This is probably less than ideal, since setting the properties later will
recompute from frames values. It is, however, slightly more rightish :)
Seemingly correct cases (i.e. where length_beats is correct):
* Recording a new MIDI region
* Splitting said region
Broken:
* Loading a region from XML
Untested:
* Import
git-svn-id: svn://localhost/ardour2/branches/3.0@8324 d708f5d6-7413-0410-9779-e7cbd77b26cf