edit-dialog: added stub code

The code added make the dialog show up. Probably anything else works,
but it's start to keep the project building while GcalManager migration
is finished and new GcalEditDialog is built.

src/gcal-manager.c

@@ -863,15 +863,15 @@ gcal_manager_get_default_source (GcalManager *manager)
 }
 
 gboolean
-gcal_manager_get_source_readonly (GcalManager *manager,
-                                  const gchar *source_uid)
+gcal_manager_is_client_writable (GcalManager *manager,
+                                 ESource     *source)
 {
   GcalManagerPrivate *priv;
   GcalManagerUnit *unit;
 
   priv = gcal_manager_get_instance_private (manager);
 
-  unit = g_hash_table_lookup (priv->clients, source_uid);
+  unit = g_hash_table_lookup (priv->clients, source);
   return e_client_is_readonly (E_CLIENT (unit->client));
 }