Support xdg-dir spec for user.conf

[saccarosium]

Hi,
This PR makes tcc put is configuration under the appropriate spot in the xdg-dir spec. It will set it only if either XDG_CONFIG_HOME is set or if .config exist.

[jrom99]

Doesn't this uses ~/user.conf or XDG_CONFIG_HOME/user.conf instead of ~/.tcc/user.conf or XDG_CONFIG_HOME/tuxedo-control-center/user.conf?

It should also prefer ~/.tcc if it exists, and otherwise create XDG_CONFIG_HOME/tuxedo-control-center so that it maintains backward compatibility