Improve notebook UI/UX

[msujew]

After merging #12442, there are still quite a few issues to be tackled related to the UI/UX of the notebook support:

• files.autoSave does not seem to be respected by the notebook editor widget/model.
• Closing a dirty notebook editor widget does not prompt to save the widget.
• Switching file-icon themes is not respected by the notebook
• Improve notebook scrolling:

notebook-scroll.mp4

• Improve toolbar items for the notebook Improves style and titles of toolbar items in notebooks. #13334
  • Items are not rendered similarly to the rest of the application, rounded border
  • The tooltip for the items do not display their full name similarly to VS Code
• "Select Another Kernel" should allow to go back in the quick-input prompt

Feel free to use this as a general tracker, and add more issues here.

[jbicker]

Improved the toolbar items.
Will look into the scrollbar issue now.