XWIKI-22351: The autosave triggers a merge conflict sometimes even if there's...

XWIKI-22351: The autosave triggers a merge conflict sometimes even if there's no save done outside the realtime session
* When replacing empty paragraphs with empty lines, take into account the CKEditor temporary elements (like the hidden selection) when determining if an empty paragraph is at the start / end of the edited content, otherwise we get conflicts in the realtime session (e.g. because the widget selection is not synchronized and so some users will have the hidden selection element while others won't)
* Disable the action buttons while an action takes place (e.g. saving) in order to avoid triggering the same action multiple times
* Rewrite the realtime auto-saver to improve the synchronization between clients when saving or handling merge conflicts