Add the list of opened shared documents to the user awareness

[brichet]

Keep track of all opened documents in the collaborators panel.

Related to #284

The file list is an panel under the user in collaboration sidepanel. The appearance of the file list is similar to that of the filebrowser (thanks @martinRenou for the idea).
The document opened in the main area of the collaborator is highlighted with a dot in front (as active document), similar to the running kernel dot in the filebrowser.

Some warnings/questions:

• it uses the HTML classes (and CSS rules) from the filebrowser. If the filebrowser extension is not loaded, it may not work properly (a remix application for instance). Don't know if we should duplicate the CSS rules.
• to retrieve the "active" document, we still use the statedb -> layout-restorer -> main, and look for :RTC: in the filename:
  • it works only if the document widget is tracked by the LayoutRestorer
  • it should be updated if :RTC: is removed from the filename

[github-actions]

👈 Launch a Binder on branch brichet/jupyter_collaboration/collab_open_docs

[martinRenou]

LGTM!