* 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.

- Change the info message "Add Images from Customize menu" to
  "Add Images" and make it clickable

- Reorganize snippet options menu

- Improve the image dialog to be able to select/unselect

- Make sure previews are adapted when changing gallery images

There was a bug with the icon edition when following these steps:
- Open the Media dialog to edit an existing icon
- Hit the "save" button in the dialog without choosing the icon
- The edited icon disappears

This happenned for icons whose original values were aliases of the
related icon. Indeed, following these steps to edit an "fa-facebook-f"
was leaving the icon unchanged as expected but doing it for a
"fa-facebook" icon made the icon disappeared. This is because the
last one is an alias of the first one and that the media dialog was
ignoring aliasies when saving the original value.

opw-725813