Clean up MidiPlaylist::read, kill copy-paste code.

Clean up MidiPlaylist::read, kill copy-paste code.

Towards putting more advanced logic here, where two copies of everything will
get even more hairy.

The two cases of reading from one or many regions are not very different in the
read phase, the only difference is the target.  So, point a reference to the
appropriate target, use the same read code in either case, then sort/etc
afterwards only if necessary.