Minor Keys in AlphaTex

[jonaro00]

Proposed feature

AlphaTex currently allows defining major key signatures with \ks G. I propose also allowing minor keys by simply expanding the switch-cases in AlphaTexImporter.parseKeySignature.

What the syntax would be is up for discussion. My idea is to keep the default behavior for major keys, using \ks Eminor for minor, but also allowing \ks Gmajor for consistency.

The switch-case would then look like:

...
case 'g':
case 'gmajor':
case 'eminor':
    ...

[Danielku15]

Sounds like a good extension to me. Syntax is also good. Added #875