libmm-glib,modem-helpers,mm-base-sim: implement Sim.PreferredNetworks

The ModemManager1.Sim.PreferredNetworks property contains the preferred
networks (and access technologies, if available) configured to the
SIM card.

This commit implements preferred networks reading with AT+CPOL.

src/mm-base-sim.h

@@ -150,6 +150,14 @@ struct _MMBaseSimClass {
     /* Signals */
     void     (* pin_lock_enabled) (MMBaseSim *self,
                                    gboolean enabled);
+
+    /* Load preferred networks (async) */
+    void  (* load_preferred_networks)          (MMBaseSim *self,
+                                                GAsyncReadyCallback callback,
+                                                gpointer user_data);
+    GList * (* load_preferred_networks_finish) (MMBaseSim *self,
+                                                GAsyncResult *res,
+                                                GError **error);
 };
 
 GType mm_base_sim_get_type (void);