app: Expose world clock locations to gnome-shell
Currently gnome-shell integrates with Clocks by picking up our settings and showing the time of the "World Clocks" locations in the calendar drop-down. However that only works when gnome-clocks is installed on the host, but not from flatpak, as the setting is "hidden away" in a container in that case.
To address this, export the relevant information over D-Bus instead, so that gnome-shell can pick it up without poking at our GSettings schema.