feat(core) Big refactor of dialog system

This replaces save_object and execute methods by 3 methods:
readPost - Reads POST data
update - Update object state
render - Render HTML UI

The point is to avoid reading POST and rendering HTML when this is not
 needed (when doing stuff through the webservice for instance).

It’s also more consisent across FD with all classes handling some kind
 of dialog implementing the new interface FusionDirectoryDialog which
 makes sure these 3 methods are implemented.

issue #6072