Add minor keys

[jonaro00]

Issues

Fixes #875

Proposed changes

Adds the ability to specify minor keys in alphaTab. Major keys can also be explicitly stated.

Checklist

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

Further details

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

[Danielku15]

@jonaro00 C#/Kotlin builds are broken by this change.

[jonaro00]

I think I managed to (poorly) implement toThrowError in C#. Kotlin is a bit trickier. Would appreciate help on that one.

[Danielku15]

Adapted the missing bits on the C# and Kotlin code to make them happy on your new ToThrowError usage. Thanks for the contribution 😁