Add support for "spectra for each note" operation

[atsushieno]

Right now spectra operation is valid only as a one-shot operation or a simply-repeating operation regardless of note on/offs. It is often very convenient to fire spectra on every note on (such as spectrum modulation changes by note on).

[atsushieno]

Some ideas:

• add "__ON_MIDI_NOTE_ON" operation and "apply" any "registered spectra" operators.
• add "__BREAK" operation that breaks the rest of MML inside a macro (or track, if doable).
• at "__ON_MIDI_NOTE_OFF" that triggers __BREAK above, within existing spectra MML definition.

Needs some consideration on how to "register" spectra. A simple and dumb idea is to use __LET to certain variables.

[atsushieno]

related but not mandatory: we need MPE support

[atsushieno]

After examining MPE specification, I'm not going to give high priority on that. It is only to provide pitch bend (and its sensitivity) per note. (CC74 is supported too, which has nothing to do with any standard MIDI device, and CAf can be just PAf in the traditional messaging.)