Respect and utilize XDG base directory specification

[GreenLunar]

Data, such as duke3d.GRP, may also be under ~/.local/share/duke3d/ (not necessarily under /usr/share/games/duke3d/). Configuration should be under ~/.config/duke3d/. Cache should be under ~/.cache/duke3d/ (e.g. crash reports, logs etc.).

See http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html https://ploum.net/207-modify-your-application-to-use-xdg-folders/

[fabiensanglard]

That would be a nice addition indeed.