Replaced Dropbox Sync with improved Import and Export functionality.

- Import from Photos (for PhotoScore) and NotateMe and MusicXML files from iCloud and other apps & cloud services.

- Export to iCloud and other apps & cloud services.

Voice allocation improvements, particularly for 3 and 4 voices.

Other parsing improvements.

Hold down play button in editing mode to toggle between playing back all instruments and just the current instrument.

Mute button fix.

UI tweaks.

Bug fixes.