Add keyboard shortcuts

It could be a nice feature to the user to provide shortcuts for common activities:

- Control-S as Save
- Control-N as New model
- Control-Z/Y as Undo/redo
- etc.

Note that care will have to be taken not to interfere with the usual input state machine for AToMPM

The issue names keyboard shortcuts for Save, New model, and Undo/redo, and warns about the usual input state machine. Start by locating that input state machine and the handlers for these activities. Done means the listed shortcuts work without interfering with normal input.

Written by the indexing model from the issue text.