XDG Base conformance on Linux

[Herdinger]

Since it's a standard and nobody likes cluttered home directorys it would be nice if the project would adhere to XDG Base Directory.

There shouldn't be to much code changes just a checking of an env var and defaulting to a directory on linux

[felixsanz]

bump

[ShayBox]

bump

[DanMan]

To be specific: instead of putting the config files into ~/.emulationstation they should be put into $XDG_CONFIG_HOME/emulationstation or if that variable is empty into ~/.config/emulationstation.

For backwards compatibility you could first try the old path and use that, if it exists, but if it doesn't you (create and) use the new one.

.emulationstation seems to appear in only <10 files. I'd do it myself but I'm not familiar with C++. Should be pretty easy task for anyone who is though.