Bug Fix: restarted assisted performances don't crash

Track.setToFirstStartMarker() now re-sets all
midiObjects after the start marker to start at their
beginnings.
Improved solution for previous bug fix

MidObject.setToStartAtBeginning() is now called
immediately when leaving the midiObject during
the performance. MidiObjects no longer have to be
reset between performances. this minimizes the
time at which MidiObject.currentMoment can be
null, and probably simplifies the implementation
of repeating chords...
Mono1 now working correctly in Song Six.

NoteOn and NoteOff handlers now work as they
should in Song Six. Has been tested with different
positions for the start and end markers.
Known bug: endMarkerPositions should only be
possible where the live performer has a chord or
rest. Currently, I think it is possible for them to end
up at the end of a system where this is not the case.