Make midi_mappings also cover MIDI note-on events.

[dpwe]

This takes the midi mapping structure built to automatically convert MIDI Control Change messages into wire commands, and extends it to work for MIDI note-on message. By default, MIDI note-on messages have the mapping -1,0,0,1,0,i%in%nl%v, meaning that any note is converted to a note-on command for the matching synth. But this command can be overridden e.g. amy.send(synth=1, midi_note_cmd='-1,0,0,1,0,i%in%nl%vZ-1,0,0,1,0,i2n%nl%v') would convert any MIDI note-on command on channel 1 to two internal note-ons, one to synth 1, and a simultaneous one to synth 2.

[dpwe]

WARNING: This modification removes the setting of e.note_source = NOTE_SOURCE_MIDI;. Whatever relied on this (I think, echoing midi in to midi out) will now be broken.