add keyboard shortcuts for navigating between tabs, frames, splitting frames, etc.

[williamstein]

We have no keyboard shortcuts at all for navigating between cocalc tabs, cocalc projects, frames, splitting frames, maximizing, etc. It's probably hard to implement that, but at least it would be nice to come up with a proposal for what the keyboard shortcuts should be. Any ideas?

• switching between open projects
• switching between open tabs in a project
• switching between frames in a single frame tree editor
• splitting a frame
• maximizing a frame