Organ ...MIDI export

When exporting Organ as MIDI file,
the result is -one big track- which included all three staff.

Would be best, because the nature of the instrument,
to get result like -three separate track-, one per staff.

Much more manageable for further treatment.


I expect we will be able to introduce options for this in future. For now, the MIDI export uses the same approach as playback itself, which will use a single endpoint for the whole instrument.