The control panel now holds an "AudioEditor" to adjust volume
and open a window to alter device settings. Audio output is definitely
working, but different buffer sizes make it sound really weird. The
same problem is present in the Juce demo app, however, so I don't
think it's a problem inherent in the GUI.

Changes to the DataThread, SourceNode, and UIComponent classes allow data
threads to disable callbacks if they are no longer receiving input. So
far this has only been tested with the Intan Board, but it works really well.
Unplugging the board in the middle of acquisition allows the thread to exit,
and callbacks to be disabled, without any seg faults.