Fix for issue #24339
Slurs between voices will now save and restore properly.
The MuseScore debugger has not been fixed to display the cross-voice
property of such slurs.
Corrected both reading and writing.
A slur is not equivalent to most of the other spanner elements since it
is related to specific voices, not once but twice.
Remove references to "this"
Updated compatability test for slurs.
Read prior version mscx files correctly by defaulting to single voice
slurs
Merge pull request #661 from MarkRS-UK/24339SlursFromOneVoiceToAnother

24339 slurs from one voice to another