Use NULL for generic marshallers in g_signal_new()

glib will use the correct marshaller automatically. And as a side
effect, we also get all glib optimizations, like a va marshaller.

gtk/gtkeditable.c

@@ -176,7 +176,7 @@ gtk_editable_base_init (gpointer g_class)
 		    G_SIGNAL_RUN_LAST,
 		    G_STRUCT_OFFSET (GtkEditableInterface, changed),
 		    NULL, NULL,
-		    _gtk_marshal_VOID__VOID,
+		    NULL,
 		    G_TYPE_NONE, 0);
 
       initialized = TRUE;