Data migration when embracing USE_XDG=1

A careful study of the source code, revealed that there is an undocumented build-parameter called `USE_XDG=1` which does exactly what is says on the tin.

This is a bit inconvenient, since our current package relies on `+set fs_homepath ~/.var/app/com.github.iortcw.iortcw` to make sure that IORTCW writes to the right directory.

### Future change
```
# For historic reasons, single- and multiplayer are different binaries
- cd /run/build/iortcw-source/SP/ && USE_XDG=1 USE_INTERNAL_LIBS=0 COPYDIR=/app/bin/ make -j${FLATPAK_BUILDER_N_JOBS} copyfiles
- cd /run/build/iortcw-source/MP/ && USE_XDG=1 USE_INTERNAL_LIBS=0 COPYDIR=/app/bin/ make -j${FLATPAK_BUILDER_N_JOBS} copyfiles
```

### Impact
this moves the main from
`~/.var/app/com.github.iortcw.iortcw`
to
`~/.var/app/com.github.iortcw.iortcw/data/iortcw/main`

which is not insignificant. We'll have to see if this is worth the hassle. It will remove the dependency on `+set fs_homepath` though which might in the long run be beneficial.