standards: Follow the XDG Base Directory Specification

[DC7IA]

Everybody likes standards in software, right?

So why not follow the XDG Base Directory Specification?

details: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

Meaning Xastir should look for (and place) its configuration files by default under a directory in /home/$user/ instead of using its own path. Usually this is .config, but this path should be derived from variables, as it can differ.

This leads to a less cluttered home directory.