[MusicXML] Support alternate volta text

To reproduce:

1) Create a score with voltas

2) Give the second time volta alternate text of Final for example

3) Export as uncompressed musicxml

4) The volta will have been exported as something like <ending number="2" type="start"/> when it should be exported like <ending number="2" type="start">Final</ending> (note that the latter imports correctly to MuseScore)

I've uploaded a musicxml file which currently imports correctly, but loses the alternate text on export.