We can't seemingly map velocity, so use the buttons for scale key.

Push the velocity UI down from `InstrumentComponent` into each of
`PianoComponent` and `DrumKitComponent`.

Then change the velocity buttons into scale selector keys in the piano mode:
* The first 7 track buttons select major keys starting from C
* The eighth becomes the layout toggle function: using the piano component
  selector button for this was buggy
* Minor keys can be had by pressing the scale button twice: C goes to C sharp, etc

Since the brief layout includes duplicate octaves, override `_install_forwarding`
and disable a seemingly-harmless assert.