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/stack.h

@@ -383,4 +383,6 @@ GList* meta_stack_get_positions (MetaStack *stack);
 void   meta_stack_set_positions (MetaStack *stack,
                                  GList     *windows);
 
+void meta_stack_update_window_tile_matches (MetaStack     *stack,
+                                            MetaWorkspace *workspace);
 #endif