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/meta/window.h

@@ -237,4 +237,6 @@ void meta_window_begin_grab_op (MetaWindow *window,
                                 gboolean    frame_action,
                                 guint32     timestamp);
 
+gboolean meta_window_can_close (MetaWindow *window);
+
 #endif