Add support for controlling the audio using a MIDI controller.

Add support for controlling the audio using a MIDI controller.

This allows the user to control the audio using a dedicated controller,
as opposed to the mouse, which generally is a more direct and intuitive
user interface.

This is rather raw at the moment (e.g., hardcoded MIDI address, no real UI
indications of bank switches, and no UI for editing the mapping), but the
required basics are generally in place.