rework snap

rework snap

snap now fills in a struct (MusicFrame) which contins a snapped frame
along with a music divisor.
this gives useful information wrt magnetic snap which may or may not
have rounded to an exact musical position.

region position may now be set musically (using quarter notes for now).

this patch fixes several problems in the current code:

	- dragging a list of music-locked regions now maintains correct
	  musical offsets within the list.

	- splitting regions using magnetic snap works correctly (#7192)

	- cut drag should now work correctly with magnetic snap.

	- musical length of split midi regions is no longer frame based.