Comply with XDG Base Directory specification

[kynikos]

I think by default the file paths used by Qtile should respect the XDG Base Directory specification.

The specification was only partially followed, and this series of commits tries to complete the implementation.

The optional dependency on PyXDG has been made a requirement, so packagers should be informed and accordingly update their scripts.

As noted in CHANGELOG, some files' default paths have changed. Users may be instructed to move .qtile_history to ~/.cache/qtile/prompt_history in order to preserve the history of commands typed in the prompt. They may also be advised to delete the old ~/.qtile.log file and refer to ~/.local/share/qtile/qtile.log, or define log_path in their config.py.

The GoogleCalendar widget seems to be still using a ~/.credentials folder, but I don't use it and I haven't tried to update it because I wouldn't be able to test it.

Last thing, I'm not sure if %changelog in rpm/qtile.spec should be updated too.

[flacjacket]

So I've been sitting on this one, and I think I wouldn't mind moving default file locations around, but I feel like having to leverage PyXDG is overkill for as few files as we generate. What do you think about just changing default file locations without adding the dependency?

[kynikos]

Thanks for reviewing this patch! No problems with doing without pyxdg of course, I've modified, rebased and repushed the branch, please see if you like it now.
I've also added a small commit to update the docs on the testing dependencies, I didn't want to open an entire PR only for that.

[flacjacket]

Great, thanks! Just pushed this batch in!