Add support for Ultrastar Format version 1.1.0, 1.2.0 and future 2.0.0

[rubends98]

https://usdx.eu/format/#specs

Add support for Ultrastar Format version 1.1.0, 1.2.0 and future 2.0.0

At the moment when using #audio instead of #mp3 as an audio tag, it doesn't recognize the audio file as already linked with the text file.

[e513]

See change log for version 2.1.0:

• Support for UltraStar song format versions up to 1.2.0
  • Tags required by the format version are read/written to the song file, but converting between different format versions is not yet supported

And yes, if only #audio is used, the song file is marked as missing in the according column.
Should this maybe be tracked in a new issue?

[complexlogic]

I agree that full support for #AUDIO is needed, and will try to find the time to implement it soon.