Replace SuperTux audio with wstsound (depends on feature/c++20)

[Grumbel]

This WIP is replacing SuperTux audio subsystem with wstsound. The wstsound library is derived from Windstille audio which itself is was derived from an older version SuperTux audio, so the code is mostly the same, but with a few new features (Opus, modplug, mp3 support, OpenAL effects). It's not fully synced yet with SuperTux audio, so there is also features and bugfixes missing.

If desired I can rename the library to staudio and move it over to SuperTux github.