Fix 19021 17630

Fix wrong direction of broken beam segment in {longer-dotted note + shorter note} groups.

See issue tracker: http://musescore.org/en/node/19021 and http://musescore.org/en/node/17360

The code is shorter and simpler than the initial proposal in the issue tracker itself. The fix passed all the tests I could think of, but it also seems 'too simple to be true', then I am NOT going to merge it until someone of the core developers reviews and approves it.