- Clearly show margins' size on editing

- Top margin icons/cursors: by an user point of view, adding a
  margin-top will move the content 'down', so change handler's icons
  and cursors accordingly

- Highlight handles during editing and if previously edited

- Vertically center handle's icon if enough room is available

In carousel snippet, the target element for background snippet option is
being changed manually. So for the new target, we have to rebind the
associated events that were bound in `start` function of the snippet
option. For that, some code is extracted from the `start` function and
is used to rebind necessary events for new targets.

In particular, this was preventing the carousel slides' backgrounds to
be automatically removed when a background-color was selected.

* web_editor, website_blog

Since recent refactoring, web editor dialogs automatically trigger
an event when closed (saved or cancel). The problem is that there
still was manual trigger of those events or variants ("save" instead
of "saved").

To prevent multi-saving and be consistent, all save events are now
automatically triggered on save/close (no more manual) and the event
name is "save".

website -> web_editor so that they are available for mass_mailing.

To allow setting margins on td elements:

As the above mentionned option allows to control the mt and mb classes
on the mass mailing items, the problem is that we would ideally want it
to be functionnal on td element as they are the main part of a mail.
As they don't support margins, there were many options to fix the
problem:
- Add more JS to place the mt and mb classes on children of td element
- Place the option on children of td element and not td element itself
but in that case, the width control of td would imply a lot of parent
option hierarchy, or the width control had also to be adapted to control
children
- Control padding instead of margin. This forces to add lots of
css rules like pt4 pt8, ... (or td.mt4, td.mt8, ...).

The last solution has been chosen. Padding is more supported than
margins in mail client anyway and the css rules will be isolated in
the mail editor anyway. Using td.mt4 instead of pt4 had the advantage
to not require any JS modification.

* Add the ability to define palettes custom selection.
* Add the ability to define a custom title for the colorpicker dropdown
* Define a custom layout for single (unique) palette layout

* Define overlay color options based on new colopicker for
Cover and Banner text containers

* Define default palettes

* Add a "transparent" button
* Add a special background for transparent background buttons

+ Some minor colorpicker design change