Add a control to modify the number of voices

[EricGeorge]

This will give a way to easily switch between 1 and many voices for testing purposes.

• This shouldn't be automatable, so it should probably be added to the SynthSound and have an ActionListener on the Synth to reset the voices when it changes.
• UI should be on the ControlBar.
• Add it to the state serialization so it gets saved/restored with the state. Will probably need to add a way to have the Synth and SynthSound serialize too.