Add a transposition controller

[broadwell]

The UI component for the controller is just a slider with ticks that go
-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6
With the label Transpose (half steps)

The implementation of the effect is more involved, though. The value of the transposition control should be applied to every MIDI note event coming from the roll player, adjusting its MIDI number value before this is sent to the ToneJS piano to be played; and the keys that appear as pressed on the on-screen keyboard also should change accordingly.