Exact beat - provide audio->music mapping for region split.

Exact beat - provide audio->music mapping for region split.

	- for those not in the know, this series provides a way to
	  remove the temporal distortion introduced when using an
	  audio frame-based gui for music-locked objects.

	  In short, the gui uses an audio frame representation to move
	  objects. It displays the object using frame_at_beat(), quantizing
	  the time value to audio frames. This is fine until the user selects
	  that frame but expects it to be interpreted as a beat.
	  Thus beat_at_frame() would not produce the user-expected beat
	  (temporal quantization error of up to 0.5 audio samples).
	  This is one method of mapping audio time to music time accurately.