[Feature Request] Linux: use XDG directories

[twolife]

Hi, Currently, the configfile on linux is saved in $XDG_DATA_HOME/Outrage Entertainment/Descent 3/ (as returned by SDL_GetPrefPath()), that's cool !

In addition to that, it would be nice if:

• the pilote files (*.plt) could be saved in SDL_GetPrefPath() too
• ./custom/cache/ could be relocated to $XDG_CACHE_HOME/Outrage Entertainment/Descent 3/
• screenshots could be saved in XDG_PICTURES_DIR (not an env variable, it's defined in ~/.config/user-dirs.dirs, there is code that can be cut&past from https://gitlab.freedesktop.org/xdg/xdg-user-dirs/-/blob/master/xdg-user-dir-lookup.c to handle that)

Thanks!