Adds support for chords in MusicXML importer

[coluzziandrea]

Issues

Fixes #667

Proposed changes

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

[coluzziandrea]

Hi @Danielku15, could you please take a look at this PR? 😊

[Danielku15]

Thanks for preparing this change and sorry for the delay. I currently have not much time to work on alphaTab.

Can you maybe also take care that the C# variant compiles?

[coluzziandrea]

@Danielku15 I've addressed comments, take a look when you have time please

[Danielku15]

@coluzziandrea Thanks a lot for your contribution. I took care of the remaining updates and cross compilation bits.

[goodgame365]

But after the update. the exported GP format file cannot be opened in Guitar Pro 8. When I try to open it, Guitar Pro 8 crashes.