This is an example of a custom Performer whose performance is affected by
graphic user input.

You turn on or off the Performers with the switches and change the speed and
range of the Performers with the sliders.

This example illustrates how to run the Music Kit performance in a
separate thread from the Application.  This helps the music to remain
rhythmically accurate, when the user is making changing with the
graphic interface.  Note that in a separate-threaded perormance, all
Music Kit method invocations must be bracketed by 
[Conductor lockPerformance] and [Conductor unlockPerformance].
