tiling: keep track of the monitor where a window was tiled

meta_window_get_current_tile_area() computes the area where the tiled window
should be based on the current pointer position but that's only meaningful
when the user is actually dragging the window.

When running the tiling constrain the pointer might be on other monitor and at
that point the window jumps to this other monitor.

https://bugzilla.gnome.org/show_bug.cgi?id=642580

src/core/window-private.h

@@ -129,6 +129,7 @@ struct _MetaWindow
    * this is the current mode. If not, it is the mode which will be
    * requested after the window grab is released */
   guint tile_mode : 2;
+  int tile_monitor_number;
 
   /* Whether we're shaded */
   guint shaded : 1;