main: Export location information to gnome-shell

Currently gnome-shell integrates Weather by picking up our settings and showing a corresponding forecast. However that only works when gnome-weather is installed on the host, but not from a flatpak, as the setting is "hidden away" in a container in that case.

To address this, export the relvant information over D-Bus instead, so that gnome-shell can pick it up without poking at our GSettings schema.

Related: gnome-shell#1158 (closed)