Add support for chord diagrams in MusicXML

[Danielku15]

As reported here alphaTab does not support reading the individual frets and strings from the MusicXML harmony definition. We can use the information from the frame element to fill up all strings and frets of the chord accordingly:

https://w3c.github.io/musicxml/musicxml-reference/elements/frame/
demo.xml.txt

[acoluzzi]

Hello there!
Is this issue still valid? I'd like to give it a try

[Danielku15]

Hey there. Yes the work item is still valid. 😉 Feel free to give it a try. Just reach out here if you need any assistance.