Proposal: Keyboard driven piano roll mode

### Enhancement Summary

A keyboard mode for editing in the piano roll - either enabled by default, turned on via config, or a specific tool.

* Includes a "point" in the pitch/time grid, movable by simple arrow keys
* Inputting a note via MIDI or the computer keyboard would result in a new note at the current "point"
* Ability to "grab" notes and move/resize/adjust parameters with modifier keys
* Keyboard-driven selection / multi-note editing

#### Justification

* Accessibility for people with limited ability to use the mouse
* Faster and more accurate note-programming workflow
* Faster and more accurate editing interface
* The "point" could be used as a default location for other features, and is fairly intuitive as a "place where *you* are" marker

#### Mockup

If there is interest, I will work on a mockup specifically with the LMMS piano roll, including choosing specific keys for specific functions, but in the meantime I have a general interactive demo here:

http://files.jneen.net/ui.html

No source files, tests, or entry points are named. Start by reviewing LMMS's piano roll and the linked interactive demo, then clarify the keyboard layout and interaction scope. Done should cover a movable grid point, keyboard or MIDI note insertion, note manipulation, and keyboard-driven selection.

Written by the indexing model from the issue text.