make monophonic MIDI groups #242

for monophonic keyboard uses, there should be Voicer-like ElementGroups and MIDIFuncs:

• MIDIFuncs listen to whole channel (or given range of numbers)
  on noteOn:
• if no held note, do startfunc (e.g. start synth/proxy)
• set note value to latest noteOn
• set amp value to latest velocity value
• keep noteOn notes in order of arrival
  on noteOff:
• remove released note from held notes
• when latest noteOn is released, set note value to last-but-one note
  if no more held notes, do endfunc (e.g. end synth/proxy)