Improve file event handling in side panel

[brichet]

Follow up #259 (#259 (comment)) to dispose of the widget in side panel when a file is deleted.
Related to #239

Changes

• Do not rely anymore on the event system of Jupyter to update the chat list in the side panel, but rather on the signal of the content manager.
• dispose of the chat widget (in the side panel) if the file is deleted. This automatically removes the chat from from the restorer, which fixes Error opening chat: not a valid path #239.
• avoid updating the file list every time the side panel is shown, only when it is attached.

[github-actions]

👈 Launch a Binder on branch brichet/jupyter-chat/handle_deleted_file_sidepanel

[andrii-i]

Looks good and prevents JupyterLab from considering chat widgets created based on deleted chat files active and attempting to restore them resulting in a file not found error / warning. Thank you @brichet.