[MusicXML] [1.1 / trunk] Problem importing tuplets from XML in a multi-voice context

Tuplets import incorrectly from MusicXML if the are interrupted (temporarily) by a switch to another voice.
Set to critical, as it leads to data corruption in MuseScore 1.1 when trying to delete the measure containing the tuplet. Even undo does not work correctly anymore in this case.
Note that the 0.9.6 branch and the trunk behave slightly different, but incorrect.