[MusicXML] Visibility of objects
The support for MusicXML's "print-object" attribute seems a bit arbitrary. For some elements it simply works, on others neither export nor import works.
Currently it does not work on clefs, which should be an easy fix though.
It does not work on metronome elements, but this attribute was added here only in MusicXML 4.0. As the corresponding PR for updating to MusicXML 4.0 has been merged, import/export of this would be a welcomed feature.