[PATCH] Adhere to XDG Base Directory Specification.

I'm not fond of this standard. So basically, instead of having all the files I need to copy a configuration from one system to another in only one directory (~/.config/decibel-audio-player), everything should be split among three directories, including a cryptic ~/.local/share?

On the other hand, it is really nice for doing backups: simply exclude .cache and you are done.
I agree that XDG_DATA_HOME has a strange default path, but one can always change that.

In any case, .config should not be hardcoded.

Just a small update; the previous patch contained a typo.