Introduce geolocation support.

Use geoclue service to obtain longitude and latitude data. Reverse
geocoding with geocode-glib provides country code. With this info we
query a city location in the libgweather database. The found location
appears automatically in the main view if no other locations within the
same country and timezone are present.

https://bugzilla.gnome.org/show_bug.cgi?id=702403
Do not allow selection for automatic locations
Add gsetting to toggle geolocation support