core: use NMActiveConnection objects throughout activation paths

They are the basic class that tracks active connections, and we're
going to use them for connection dependencies.  So use the fact that
both NMVPNConnection and NMActRequest have the same base class
instead of using object paths.

src/nm-activation-request.h

@@ -52,7 +52,8 @@ NMActRequest *nm_act_request_new          (NMConnection *connection,
                                            gboolean user_requested,
                                            gulong user_uid,
                                            gboolean assumed,
-                                           gpointer *device);  /* An NMDevice */
+                                           gpointer *device,  /* An NMDevice */
+                                           NMActiveConnection *master);
 
 NMConnection *nm_act_request_get_connection     (NMActRequest *req);