Suggestion: add XDG directory support

[red-rj]

Hello there

I did a similar PR for yquake2 and figured I should ask if you guys want it

I want to check for user files in "$XDG_DATA_HOME/Quake III" or similar, if ~/.q3a already exists use it to keep compatibility

[ccornix]

This feature would be awesome! See also a similar PR for ioquake3:
ioquake/ioq3#792

[ensiform]

More obfuscation of filesystem scattered in multiple locations isn't really a benefit just because a few people decided it was a standard doesn't really make it a good choice. I'm fine with allowing the xdg data home instead of $home but splitting up like the ioq3 or did was an utter nightmare.