[MusicXML export] System-crossing clef change not exported
Steps to reproduce:
1. Create a score that contains a clef change at the end of a system.
2. Export the score as MusicXML.
3. Re-import it into MuseScore (or other scorewriter)
The clef change is gone.
Using MuseScore 2.0.1 on Windows 8.1.
(not sure if this should be higher priority than "normal"...)