[trunk] Sub-beam on wrong side for downstem notes

Steps to reproduce bug
1. Create a new score
2. Enter two 16th notes followed by an 8th note
3. If the group of notes is upsteam, press X to make it downstem

Expected behavior: The 8th-note beam should be on the outside

Actual behavior: The 16th-note sub-beam is on the outside. See
http://musescore.org/sites/musescore.org/files/incorrectbeams2.png

MuseScore version: r. 4003 nightly build

(Operating System: Windows 7)

first reported by newsome: http://musescore.org/en/node/9218