[MusicXML] [1.1 / trunk] Import of tuplets containing chords is incorrect

When importing a MusicXML file with a tuplet containing chords, each chord except the last one is added multiple times to the tuplet.
In the MuseScore 1.x series, this may lead to:
- corruption in the next measure when the measure containing the tuplet is deleted
- note that this cannot be undone, as the undo administration will be corrupt too !
- crashes editing the tuplet
In the trunk, the issue results in corruption when the tuplet is deleted: an empty space results, instad of a note or rest.