[MusicXML export] Add placement attribute in tuplet at MusicXML export

Upon exporting to MusicXML, the placement attribute (below/above) within the tuplet element is not exported. (This would be a great little feature for interoperability.) When re-importing it into MuseScore (or other notation software), the number placement is reversed (apparently taking default settings).

Other attributes as bracket (yes/no) and show-number (actual/both/neither) work fine.