objects interested in being notified when the number of tracks per staff has been changed
(like the ScoreEditPanel) should implement NumTracksPerStaffChangedListener and register
itself with the score using this method
This event can be posted in the future to free a SynthCircuit allocated by a
For example, in a JMSL Instrument's play() method, you could post a free() the same
JMSL time as a noteOff(), by calling freeVoice()
NOTE: Scheduled playTime is JMSL time! But free() time is int JSyn time!!!
Not used by any JMSL classes.