NoteOffs are now only sent when necessary.

Many redundant noteOff messages are no longer
sent at the ends of MIDIChords.
The new code for sending noteOffs when they ARE
necessary at the ends of MIDIChords is called by
none of my current scores, so it has not been tested.
Such (untested) noteOffs are necessary at the ends
of MIDIChords having a chordOff, but containing
basicMidiChords that have no chordOff.

Also in this commit:
The score of Song Six and assistantPerformer.html
have been updated for new system breaks. There are
now 13 pages instead of 14.
Song Six score updated by Moritz (final version). Known Bug.

Known Bug. Starting with the startMarker somewhere in the
middle of the score fails to set patches correctly!
Bug fix: Patch changes are now sent to every chord after all.

Reversed the changes made on 21.01.2014 because
otherwise performances don't start correctly from a
startMarker placed after the beginning of a score.
The reduction in the number of NoteOff messages
seems, however, to work.
Song Six.mkss added. Score has been recompiled (Moritz).