Add a property to GNetworkMonitor indicating the level of network
connectivity: none/local, limited, stuck behind a portal, or full.
The default implementation just returns none or full depending on the
value of is-available.
https://bugzilla.gnome.org/show_bug.cgi?id=664562
Add GNetworkMonitor and its associated extension point, provide a base
implementation that always claims the network is available, and a
netlink-based implementation built on top of that that actually tracks
the network state.
https://bugzilla.gnome.org/show_bug.cgi?id=620932