Add fork to F-Droid

[licaon-kter]

ref: https://gitlab.com/fdroid/fdroiddata/-/issues/3101

Code issues: version seems to be stuck and oddly defined in two places: https://github.com/helge17/tuxguitar/blob/master/TuxGuitar-android/AndroidManifest.xml#L5-L6 and https://github.com/helge17/tuxguitar/blob/master/TuxGuitar-android/build.gradle#L15-L16 ( I recommend the latter)

Inclusion policy: https://f-droid.org/en/docs/Inclusion_Policy/ forks need a different appID than original app, and while this could be considered as continued work.. I didn't see any agreement with original dev :(