Fix issue #24468 - Tremolo exporting to parts
Completing tremolo export to part changes,
http://musescore.org/en/node/24468, adding functionality for
adding/deleting after part creation.
Merge branch '20082-PartExportFailure' of github.com:MarkRS-UK/MuseScore into MarkRS-UK-20082-PartExportFailure
fix test for tremolo in GP5 import
Merge branch 'MarkRS-UK-20082-PartExportFailure'