* Creating a new structure by transforming all the plugins in the
  library using the odoo inheritance system. Plugins are easier to
  implement with the AbstractPlugin to add Odoo behaviors.

* From now on, the methods of the library (in this case Summernote) can
  no longer be called by other modules or files. Only the wysiwyg
  widgets can access it, to simplify the updating process. The wysiwyg
  object serves as an interface.

* Depending on the options the snippets will be loaded or not, the
  editor will be in an iframe or not... all of this is transparent from
  the outside.

* Regarding iframes, all controllers related to editing have been
  removed: the new API no longer needs them. This speeds up loading,
  eases testing and removes complexity for the same
  features.

PUBLIC FEATURES

There are several public methods on the Wysiwyg class:
* Wysiwyg.prepare (WidgetParent): returns a deferred resolved when the
  library (xml, lazy, assets...) is loaded.
* Wysiwyg.getRange (DOM): returns the range (selection in the dom)
* Wysiwyg.setRange (startNode, startOffset, endNode, endOffset): creates
  a range (selection in the dom)
* Wysiwyg.setRangeFromNode (DOM, options) that creates a range from an
  element (option available to select all, start or end)

A jQuery selector was added: :o_editable, which indicates whether the
current element is editable. That is, if it is contained in a tag with
the attribute 'contentEditable = "true"' or in a tag with the class
o_editable.
Several methods are also present:
* focusIn: makes a focus and places the cursor at the beginning of the
  element
* focusInEnd: makes a focus and places the cursor at the end of the
  element
* selectContent: makes a focus and selects the content

HTML FIELD

The HTML field can receive different options:
* style-inline: {boolean} transforms a class into an inline style when
  saving and vice versa when reading.
* no-attachment: {boolean} prevents the use of attachments (in media
  dialog)
* cssEdit: {xml_id} to use a template containing the css to loaded in
  an iframe when editing
* cssReadonly: {xml_id} to use a template containing the css to load
  into an iframe when viewing in readonly
* snippets: {xml_id} snippets template (can be used with or without
  cssEdit)
* wrapper: {template} qweb static template (containing a tag:
  id = "wrapper") that will include the content during editing (removed
  on save)

MASS MAILING

A widget was created for mass mailing. There are now two fields:
body_html and body_arch.
body_arch contains the code with the class without conversion into
inline style, useful when editing and one with the inline style that is
visible in readonly mode and sent by email.
Advantage: no spreading errors, able to update css/theme, able to do
more changes when converting to inline style so that a maximum of mail
clients have an impeccable rendering.

Co-authored-by: Antoine Guenet <age@odoo.com>

With this commit, any component that needs an instance of the
root widget can simply require the module 'root.widget'. Based on
the context (backend, frontend, iframe), the real root.widget
will resolve to either the webclient, the website root instance, or
the iframe root instance.

This is desirable in order for the tour_manager to become a child
of the root widget, as tours are enabled in any of the above contexts.

This commit is a requirement in order to solve an issue with JS services,
in which there should be only one service provider at any given time.

Broken since https://github.com/odoo/odoo/commit/2972976962617d4b8a0113bae58c640ab41cdff8

* mass_mailing, payment, point_of_sale, portal, survey, web, web_tour,
  website_blog, website_crm_partner_assign, website_event,
  website_event_questions, website_form, website_forum, website_gengo,
  website_hr_recruitment, website_links, website_mail,
  website_mail_channel, website_mass_mailing, website_quote,
  website_sale, website_sale_options, website_slides, website_twitter

This commit reviews the whole "JS side" of the web_editor and website
apps. This is a first step to be able to improve them with new and
better functionnalities; this commit is not supposed to change any
visual behavior.

The main goal was to achieve a structure similar to the backend one.
Now, the frontend side also has a root widget (like the WebClient)
and all other widgets are attached to it one way or another. This allows
the benefits of using the 'trigger_up' functionnality for example.

As RPC are now mainly done with the `this._rpc` functionnality (being
possible thanks to the parent hierarchy), the frontend will also be
possible to test thanks to QUnit in a future update (besides the "text"
editor side which still requires a refactoring to be able to do that).

---

Here are some of the changes:

(-) conventions and documentation

The code has been updated to follow JS conventions and a lot of code has
been commented (around +2000 lines of comment). This also means that
lots of functions have been renamed to use camelCase or simply to make
their name understandable.
See https://github.com/odoo/odoo/wiki/Javascript-coding-guidelines.

(-) deprecated: web_editor.base

The "web_editor.base" module has been split and does not force the
modules which require it to wait for DOM ready anymore. This was indeed
slowing loading times, but also prevented to use some modules in some
contexts (see the LESS editor use in web_studio which is the subject of
another task).
Now the "editor context" can be got thanks to the "web_editor.context"
JS module with its "get" function.
The "web_editor.base" module should probably not be used anymore (see
its code and recent updates).

(-) new: web.dom_ready

If a JS module should wait for the DOM to be ready to be executed, a
new JS module has been created: "web.dom_ready". This should always
be used in a module which only want to instantiate stuff. Do not
extend (or worst, include) classes after DOM ready.

(-) website.website

The "website.website" module has been split. "website.website" does not
return anything useful anymore, it just initialize some miscellaneous
stuff, without waiting for the DOM to be ready. You might want to check
"website.utils", "website.content.compatibility" or `WebsiteRoot`. Also
`website.form` has been deleted (use `this._rpc`), so has been
`website.error`. `website.prompt` will be removed in a future update to
be replaced by `Dialog.prompt`.

(-) widgets are great

Many classes which were not widgets are now widgets. This allows them to
use the 'events', the 'xmlDependencies' and the 'this._rpc' features for
example. Here are some of the main ones:

- Snippet options: these were classes with a `$el` for the menu element
    and `$target` for the customized element. This is still the case
    but following standard `Widget` structure (one exception: using
    `this.$(...)` searches in the `$target` as before this update).

- Snippet animations: instead of class instances with a `$target`
    element which can be `start` and `stop`, these are now standard
    widgets which can be `start` and `destroy`. `this.$target` is
    an alias to `this.$el` for ease of compatibility.

- Snippet editors: instead of class instances in charge of an editor
    overlay, these are now widgets. Each "child" snippet editor is
    properly attached as a "child", which allows editors to communicate
    and to be properly destroyed.

(-) root widgets and website navbar

The frontend is different of the backend. In the backend, the page has
an empty <body/> element and all the components are instantiated from
parent to children (i.e. the `WebClient` is instantiated and is in
charge of instantiating the `ControlPanel`, etc). The frontend cannot
work like that on page loadings as they are way more frequent than in
the backend and we do not want them to flicker. A frontend page is
loaded as a <body/> element which already contains the website navbar
and its menus and the whole content page. JS code has to be "attached"
to these existing elements. This is possible thanks to the `RootWidget`
instances and the specialized `WebsiteRoot`, `IframeRoot` and
`WebsiteNavbar` (see code for details).

(-) lazy loading

No more (or at least a lot less) XML/JS has to be loaded on page
loading, thanks to the use of the `Widget.xmlDependencies` feature.
XML which have to be lazy loaded is loaded only on related Widget
instantiation if necessary, which allows to execute a lot of JS code
before the DOM is ready and to start many widgets on DOM ready (not
later). A visual benefit of this is clicking on the 'edit' button as
soon as it is possible: before this commit, this was sometimes not
doing anything as event handlers were not binded yet.

Still a possible exception: loading the session and locales. This may
be asynchronous stuff which is still required before widget
instantiations but this will be the subject of another task.

(-) deprecated code and code location

More than reviewing code and organizing it, many apparent dead code was
removed. More importantly, mislocated code was put in the right app.
This is the case for snippet animations which is a concept for website
apps but was defined in the web_editor app, or some translation concepts
which were part of website but should have been part of web_editor.

---

There are probably more things to say about this commit but I will let
the comments speak for those.

* web_editor, web_planner, website

The 'Dialog' class is used in both backend and frontend. Also, lots of
actions can be done without the use of any modal. It thus makes sense
to lazy load its related xml only when a first modal is opened.

This change also allows to get rid of the "base_common.xml" file as the
dialog template is the only remaining one in there and, in the future
website update, it will allow to not load any static XML file on website
page loadings.

Previous commit tried to handle iframe assets in a more standard way
but instead broke the order followed to install iframe communication.

While waiting for a more clean way to handle iframe assets and
communication, revert to old set up.

* Remove dead code + optimization
* Use Dialog API from web module
* Reorganize files
* Correct buggy $() function for snippet option class
* Add comments

* Fix the snippet parent navigation :

If a page contained a three level snippet structure (i.e A contains
B contains C), when editing the following bug occured :

If B or C was selected first, parenting buttons were correctly set.
But if selecting A then C, clicking on the parent lead to going back
on A instead of going on B.

* Fix snippet thumbnail layout
* Better isolate ui css from themes
* MediaDialog image list : prevent displaying empty attachments
* ...

boot.js log:

* ``Missing dependencies``:
  These modules do not appear in the page. It is possible that the JavaScript
  file is not in the page or that the module name is wrong
* ``Failed modules``:
  A javascript error is detected
* ``Rejected modules``:
  The module returns a rejected deferred. It (and its dependent modules) is not
  loaded.
* ``Rejected linked modules``:
  Modules who depend on a rejected module
* ``Non loaded modules``:
  Modules who depend on a missing or a failed module