* web_editor

Odoo extends jQuery to add some utility functions. The 'prependEvent'
one add the possibility to add an event handler on an element (like
the jQuery built-in 'on' function) but to be executed before the
already binded ones.

This function was however incomplete and did not allow to use jQuery
namespaced events.

* web_editor, web_tour, website_event_track

Before this commit, one JS file was responsible for all the library
extensions and fixes. There were two main problems with this system:

- The file was only available in the backend (as some extensions are
  made on backend-only libraries).

- Some other modules were doing library customizations.

This commit creates one file per library to regroup their associated
customization and adds them in the right asset. These are placed in
the web module and every other module should avoid doing customizations.
This allows to have libraries consistency accross all modules and
between the backend and the frontend.

Use this.rpc instead of Model object, with web.ajax if necessary.

nb: EventDispatcherMixin include now the custom_events managment instead
of widget

When editing translations, you were not able to copy/paste texts before
this commit. Indeed, in some contexts, in some browsers, copy/pasting
text was creating a new paragraph inside the translation, preventing it
to be saved.

This commit simply adds code to remove these paragraphs each time text
is changed.

A more elegant solution will be found for master with website and
editor improvements.

* 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
* ...

The translation value is given as an HTML fragment.  It is converted to a valid
XML fragment if the translation's field requires it.