feat: follow XDG spec for the configuration directory

[NotAShelf]

Closes #208

Follows the XDG specification for configuration files.

With this change, Anyrun now searches $XDG_CONFIG_HOME/anyrun (most commonly
$HOME/.config/anyrun) and then directories in $XDG_CONFIG_DIRS (defaults to
/etc/xdg iirc). We fall back to the original /etc/anyrun as a last resort
Updated plugin search paths to include XDG directories.

I've also added $XDG_CONFIG_HOME/anyrun/plugins and /etc/xdg/anyrun/plugins
to the search path.

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Ported by a stale GTK4 port branch that I was experimenting with, let me know if
you see anything left over that will cause breakage.

[Kirottu]

If we start messing with the paths, we should probably integrate changes proposed by #202 as well. System installation of plugins should indeed be in /usr/lib/anyrun but not sure if user specific plugins should be in the config folder. Any input on the user plugins folder?

[Kirottu]

This will be fixed when the relm4 work is done, merged and released.