Adds side panel widgets to the tracker

[brichet]

Add the side panel chats to the widget tracker.

Fixes #130
Fixes #139

The chats tracker tracks both LabChatPanel (the main area widget) and ChatWidget (widget in the side panel).

IMPROVEMENT

• The layout restorer is now able to open back the chats in the main area and in the side panel.
• The shortcut to focus on the chat input (Accel Shift 1) works for all the chat widgets. The last used input is focused.
• EDIT: the chat input is focussed when clicking anywhere on the chat except on a focusable element.

TO DO

• improve the tracking of the chat widget
  The widget become the current widget of the tracker only if an input has been clicked. We should focus on the input component when the chat is clicked (anywhere except on an other input). This would also be a nice UX improvement.
• add tests on it

[github-actions]

👈 Launch a Binder on branch brichet/jupyter-chat/side-panel-chats-tracker

[dlqqq]

@brichet Looks good, thank you for building this! Tested your PR locally and it seems to work well. 🎉 Just left a few extra comments which don't block this PR.