MusicXML: Implement staff changes in piano staves

MusicXML: Implement staff changes in piano staves

When we encounter a note with a different staff value than the
previous note in that voice, simply add the \change Staff=.. event
into the notes. However, this does not work inside chords, so we
ignore all staff changes inside chords. I'll have to find a different
way to print some notes of a chord on one staff and the other notes
on the other staff...