Trigger chords with keys

[ChrisWellsWood]

@SonWu requested a new feature where key presses on the performance track could trigger chords. This is possible but there are a couple of issues:

1. There could be latency as when a key is pressed, there's a round trip to the app and back to trigger the sound.
2. You can't change the octave on the performance track, which means that I'll need to intercept the midi message from the keyboard, determine the key, which is non-trivial as there's no modulus function, apply the octave modification, create the chord then send the output signal.