window: Add an accessor for whether the window can close

The shell will use this to determine whether to show a close
button in the overview.

https://bugzilla.gnome.org/show_bug.cgi?id=699269

src/core/window.c

@@ -11218,3 +11218,9 @@ meta_window_compute_tile_match (MetaWindow *window)
       window->tile_match = match;
     }
 }
+
+gboolean
+meta_window_can_close (MetaWindow *window)
+{
+  return window->has_close_func;
+}