It was always reporting true, even if disconnected. At the same time,
add a signal that is emitted when state changes and update the UI
accordingly.
In the future (with another libnm-glib API break) we should use the
NMVPNConnection object to track the connection state, so that we can
show if we're connecting or we need authentication.
https://bugzilla.gnome.org/show_bug.cgi?id=646380