monitor-manager: Rename output_id to winsys_id

The output_id is more of an opaque identifier for the monitor, based on
its underlying ID from the windowing system. Since we also use the term
"output_id" for the output's index, rename our use of the opaque cookie
"output_id" to "winsys_id".

src/core/window-private.h

@@ -166,7 +166,7 @@ struct _MetaWindow
    * that to toggle between normal/tiled or maximized/tiled states. */
   guint saved_maximize : 1;
   int tile_monitor_number;
-  int preferred_output_id;
+  int preferred_output_winsys_id;
 
   /* Whether we're shaded */
   guint shaded : 1;