Add off-screen speaker notes

[bollwyvl]

Elevator Pitch

Support an off-screen control panel for navigation and viewing speaker notes.

Motivation

When presenting with multiple screens, this is useful for making reducing UI clutter.

Design Ideas

• Initially, it should be possible split off a separate window and use dumb HTML controls (not a full lumino app) to trigger various commands
• later, this could potentially be possible with RTC, allowing for a second device (e.g. tablet) to do the work
• though maybe this is partially covered by some of the "future work" in Persist Deck position #3
• could support both the on-screen remote, as well as offscreen.
• a relatively general-purpose need, this could be configured very similarly to the toolbar registry,
• a more advanced version would be
  • on top an existing SVG/HTML, with buttons spanning selectors
  • or 2d CSS grid to support things like the "compass rose"
    • this might encourage a dependency on jupyterlab-fonts so that these are more convenient to build ahead of time in JSON.