App audio switch should play or stop its audio network instead of toggling the global audio graph

[balintlaczko]

Currently the audio switch on the app ui and the App.audio prop will trigger either graph.start() or graph.stop() where graph is the global audio graph. To implement multiple apps #3 the switch should be repurposed to instead toggle between self.audio_out.play() and self.audio_out.stop() which register or deregister the App's audio network on the graph (instead of starting/stopping the graph itself, which could have bad side-effects).