window: Introduce meta_window_get_tile_match()

Returns the matching tiled window. This is the topmost tiled window in a
complementary tile mode that is:

 - on the same monitor;
 - on the same workspace;
 - spanning the remaining monitor width;
 - there is no 3rd window stacked between both tiled windows that's
   partially visible in the common edge.

https://bugzilla.gnome.org/show_bug.cgi?id=643075

src/core/window-private.h

@@ -410,6 +410,9 @@ struct _MetaWindow
 
   /* Focused window that is (directly or indirectly) attached to this one */
   MetaWindow *attached_focus_window;
+
+  /* The currently complementary tiled window, if any */
+  MetaWindow *tile_match;
 };
 
 struct _MetaWindowClass
@@ -663,4 +666,6 @@ void meta_window_propagate_focus_appearance (MetaWindow *window,
 gboolean meta_window_should_attach_to_parent (MetaWindow *window);
 gboolean meta_window_can_tile_side_by_side   (MetaWindow *window);
 
+void meta_window_compute_tile_match (MetaWindow *window);
+
 #endif