Fix GtkApplicationWindow menubar placement

Since the menubar is part of the content, we need to give
it the content_window as parent window, to make things
work again.
https://bugzilla.gnome.org/show_bug.cgi?id=696561

gtk/gtkwindowprivate.h

@@ -69,6 +69,7 @@ void            _gtk_window_get_wmclass            (GtkWindow  *window,
 void            _gtk_window_set_allocation         (GtkWindow           *window,
                                                     const GtkAllocation *allocation,
                                                     GtkAllocation       *allocation_out);
+GdkWindow *     _gtk_window_get_content_window     (GtkWindow           *window);
 
 typedef void (*GtkWindowKeysForeachFunc) (GtkWindow      *window,
                                           guint           keyval,