Wifi and mobile broadband have signal indicators and are thus
more useful than vpn icons in the panel. Therefore, in the case
we have both wifi/3g and VPN we prefer the former as the "primary
icon" and add a lock next to it.
Behavior when VPN is added to wired or other connections is still
preserved: the wired icon is replaced by vpn.
https://bugzilla.gnome.org/show_bug.cgi?id=672591