Introduce piano and drum kit modes.

They are the shift functions of the sixth and seventh scene buttons respectively, and
turn the 8x8 button grid into playable midi instruments.

In these modes
* the scene buttons still launch scenes
* WIP: the track buttons now set midi note velocity
* the up/down bank buttons control the playable pitch range
* the left/right bank buttons control output channel.

To avoid mapping conflicts:
* UI signals are sent on channel 0 only
* Piano outputs are restricted to channels 1 to 11
* Drum kit outputs are restricted to channels 12 to 15