This patch changes the ScoreActivity (and the ScoreView that powers it)
to use a generic MidiListener rather than the old
MultiChannelSynthesizer, so that it can play with the new C++ based
sound output module.
The instrument selection maps to MIDI channel, which doesn't actually
change instruments (the synth module is not multitimbral), but this
could be made to work.
Also, the score is stored in protocol buffers, and the plan is for those
to go away, probably replaced by JSON, because of code size and build
difficulty.
New activities can subclass from the SynthActivity, and that will keep
the sound going. We'll actually do something a little different for
preferences, but this will be useful for other activities.
Raph Levien