window: add a style class when we automatically create the titlebar

To distinguish it from custom titlebars added from applications.

https://bugzilla.gnome.org/show_bug.cgi?id=706592

gtk/gtkwindow.c

@@ -5273,6 +5273,7 @@ create_titlebar (GtkWindow *window)
                 NULL);
   context = gtk_widget_get_style_context (titlebar);
   gtk_style_context_add_class (context, GTK_STYLE_CLASS_TITLEBAR);
+  gtk_style_context_add_class (context, "default-decoration");
 
   title = g_markup_printf_escaped ("<b>%s</b>",
                                    priv->title ? priv->title : get_default_title ());