The web_editor app uses a 'cssFind' function but does not expose it. It
makes sense to move it in the web app in the 'dom' utility function set.

* web_editor, web_tour

jQuery has the built-in ':visible' selector. This is however not enough
as this only checks if the element and its parents have a visible
'display' property. Ironically, if the element has the 'visibility'
property set to 'hidden', it will be considered ':visible' by jQuery.

This commit creates the ':hasVisibility' and ':hasOpacity' selectors
which respectively check if the element and its parents have a visible
'visibility' property and a visible 'opacity' property.

* mail_channel, mass_mailing, twitter, event

- t-install attribute holds the technical name of the module that
  needs to be install to use a particular snippet.

- Disable the drag & drop feature for these dummy snippets.

- Move some images to website module.

- Installed snippets display at the top of the list.

- Hide not-installed module snippets if user don't have rights to install module.

* 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.

When cloning a snippet section thanks to the editor, the associated
snippet options' `on_clone` methods are called. Now these methods
receive a new argument `options`. The option this commit introduces
is the `isCurrent` one. It allows the method to know if the snippet
section it is associated to is the one that was cloned or if it is a
child of the one that was cloned.

This is useful to solve a known bug when cloning elements which have
bootstrap columns:

- When one column is cloned we want it to lose its col-*-offset-*
  classes so that the offset is not duplicated (only the column).

- When a container with column children is cloned we do not want
  these column' offsets to be removed (we want the whole cloned
  container to be the same).