Add a new geolocation source that makes use of Mozilla Location Service[1] to
determine location based on nearby WiFi networks.

Unfortunately it also introduces a bunch of ugly warnings from
libnm-glib & libjson usage that are at the moment a bit beyond me but it
works:

** (geoclue:13207): WARNING **: Could not create object for
/org/freedesktop/NetworkManager/Settings/2: No session found for uid 101
(unknown)

** (geoclue:13207): WARNING **: handle_property_changed: failed to
update property 'available-connections' of object type NMDeviceWifi.

(geoclue:13207): Json-CRITICAL **: json_object_get_object_member:
assertion 'node != NULL' failed

[1] https://wiki.mozilla.org/CloudServices/Location