gtk3x: Avoid gtk_widget_destroy() on removal of cma shell
gtk_widget_destroy() is going away in gtk-3.98.4. Reimplement cma shell handling so that gtk_widget_destroy() is not needed with it.
gtk_widget_destroy() is going away in gtk-3.98.4. Reimplement cma shell handling so that gtk_widget_destroy() is not needed with it.