[Feature] Store application data in XDG_DATA_HOME. (Linux)

[Blisterexe]

Before Requesting

• I found no existing issue matching my feature request
• This request is related to Millennium, and not a request for a specific theme/addon

Describe the feature you'd like!

Although a lot of developers store app files in /home, the proper way of doing it is storing data in one of the XDG directories, this makes people's /home's more clean and allows for the user to change where the files are stored.

Anything else?

This is rather low priority, but is a big nice to have and doesn't seem too difficult to implement, i might be wrong though.

[shdwmtr]

I'm probably going to move everything in proper user folders soon, I just kept it the way it is as it was derived from Windows.