Fix MusicXML Slurs and Reworked Repeats

[Danielku15]

Issues

Fixes #865

Proposed changes

• Extended MusicXML Tests to perform serialization/deserialization to detect potential errors
• Fixed slur handling which caused some errors
• Reworked handling of repeats to align with correct display/playback of GuitarPro (MusicXML tests reported some problematic handling of repeat groups and repeats in general).

Checklist

• I consent that this change becomes part of alphaTab under it's current or any future open source license
• Changes are implemented
• Existing builds tests pass
• New tests were added

Further details

• This is a breaking change
• This change will require update of the documentation/website