Add new "checksource" target to the library makefiles, for automated source file existance checking.

Rename new Audio class driver functions, callbacks and events to ensure that they contain the USB mode (Device or Host) in the function names.

Correct errors in the existing Audio demo description files due to the new demo features (variable sampling rate vs. a fixed sampling rate).

Fix errors in the new Audio Host mode Class Driver, which would have prevented data from being sent or received properly by the device.

Add microphone/square wave generation compile time switch to the Low Level AudioOutput Host demo.