Reconsider left panel ranking of core extensions for 3.0

[ellisonbg]

We are making some changes to the left panel by making the command palette modal and by combining the running and open tabs panels. As part of this, we should review the ranking of left panel items we are shipping.

[ellisonbg]

Current (JupyterLab 1.0/2.0):

• Left:
  • 100: filebrowser
  • 200: running, git
  • 300: command palette
  • 400: none
  • 500: properyinspector (=DEFAULT_RANK)
  • 600: tabmanager (removed in 3.0)
  • 700: toc
  • 800: none
  • 900: none
  • 1000: extensionmanager (disabled in 3.0)
• Right:
  • 500: debugger (=DEFAULT_RANK)

Proposed (JupyterLab 3.0):

• Left:
  • 100: filebrowser
  • 200: running
  • 300: command palette
  • 400: toc
  • 500: git
  • 600: none
  • 700: none
  • 800: none
  • 900: DEFAULT_RANK
  • 1000: extensionmanager (disabled in 3.0)
• Right:
  • 100: properyinspector
  • 200: debugger

Or in terms of ranges:

• 0-500: Reserved for JupyterLab
• 501-899: Usable by third-party extensions
• 900: DEFAULT_RANK
• 1000: extensionmanager