Session management

[seeM]

Add a new "Sessions" tab to the sidebar. E.g. this is how XCode's sidebar tabs look:

https://user-images.githubusercontent.com/559360/233305223-0720da9b-3700-44a2-a9e6-9ef9480157bd.mov

The actual sidebar is a single column table with each item being a running session. For each session we could show:

• File name
• File path
• Kernel name
• Button to shutdown the session

Jupyter Notebook shows file path, kernel name, and shutdown button.

Jupyter Lab shows file name, and shutdown button.

VSCode doesn't seem to have any session manager. Maybe worth considering whether it's necessary...