Noise, random number generator and property interface cleanup

Added per-state A2_PRANDSEED and A2_PNOISESEED properties.

'wtosc' now uses the 'noisestate' field in the engine state that the
voice belongs to.

Removed the global noise state from the 'noise' wave.

Moved the property interface to properties.h.

Added a2_SetProperties(), for setting multiple properties.