network: add VPN support to the connection editor

Unfortunately, the VPN plugins provide their own .ui files for their
editor pages, so we can't make them look competely GNOME-3-ish. But
the code does try to fix them up a little bit by realigning the
labels.

vpn-helpers.[ch] is nearly identical to network-manager-applet's,
but eventually this code will move into libnm-gtk.

https://bugzilla.gnome.org/show_bug.cgi?id=691285

panels/network/cc-network-panel.c

@@ -1020,6 +1020,7 @@ panel_add_vpn_device (CcNetworkPanel *panel, NMConnection *connection)
                                 "id", id,
                                 "connection", connection,
                                 "client", panel->priv->client,
+                                "remote-settings", panel->priv->remote_settings,
                                 NULL);
         g_signal_connect_object (net_vpn, "removed",
                                  G_CALLBACK (object_removed_cb), panel, 0);