I rethought the XDG_CONFIG_DIR feature and I think it would be best if we provide a small helper:
  `xwii_config_write_mp_normalization()`
in libxwiimote which takes the 4 values and writes them to XDG_CONFIG_DIR/...
And one helper to read these values. This prevents every user from writing these helpers themself and we can also optionally add synchronization/atomicity if it turns out to not work reliably later.

But that's no requirement to get this merged. We can add that later.