Add off-screen speaker notes

Elevator Pitch

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

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

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 #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.