Implement Gordon-Smith 2-pole oscillator

+ ensure that new audio path is completely out of the way when it is not enabled (e.g., extra buffer copies)
AudioSourceTone is instantiated within Audio.h/.cpp - it is our (new) sine generator.

Regarding the 'business logic' - there is  actually very little here.   In any event,  C++ classes are actually not interfaces, rather,  they are interface *and* implementation combined - stripping out member functions into .cpp files does not alter this.  If we actually want interfaces in header files, then we should implement pure/abstract interface classes and expose these.   As long as the inline-implementation is light (it is), and the interface subset of the class is clear (it is), then I don't really see any strong issue.