midi note drags are music-based.

midi note drags are music-based.

	- wysiwyg (during drag) when dragging more than one note across
	  a tempo change.

	- introduces a muscal equivalent of snap_delta (only used for
	  note drags atm)

	- split earliest note in selection into a separate function

	- MRV::copy_selection() returns the equivalent _primary note
  	  to avoid offset hell.

	- RV::snap_frame_to_frame returns a MusicFrame

	- prevent note drag moving before region start.