Conform to XDG Base Directory Specification

[dann-merlin]

Copying from an original EmulationStation issue (Aloshi#559):

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.

[namgo]

Hey all! Just for context on the mention, I'm having a hard time packaging the application into Void Linux. Someone had previously tried to package it but likely had files already in their $HOME/.emulationstation when they tested the package.

[cmitu]

Not sure how packaging it's related to this issue though ? You probably need something like #618 implemented or manually add the necessary files (resources) under /etc/emulationstation.

[namgo]

You're totally right! That's much easier for us to implement, thank you!