Route all spatializers to buses, add output synth

This MR makes a significant, but non-breaking, change to how spatializer audio is routed inside Satie.

Changes are:

1. An audio Bus of the appropriate channel size is created for each listeningFormmat specified in the configuration. These buses are kept in the SatieConfiguration.spatBus array (much like the ambiBus array).
2. The audio output of a spatializer is sent to its Bus, not directly to its corresponding outBusIndex.
3. Post-processors use the Bus of their spatializer for input/output. This is the same as before except for using the Bus rather than an outBusIndex channel.
4. A persistent satieOutput Synth is automatically created that takes each spatBus as input and outputs their signals to their corresponding outBusIndex. The Synth gets created immediately after Satie boots, and is recreated after Cmd-period.

Also, we now make use of ServerQuit so that we call the appropriate code whenever Satie's server is quitting. This should have been done before, and now we do.

I've added a set of tests to make sure things happen as they should and signals get routed where they are configured to be routed. If you think of anything missing, please let me know.