code: func (a *Box) SetCenterWidget(child IWidget) {
  	if child == nil {
  		C.gtk_box_set_center_widget(a.native(), nil)
  	} else {
  		C.gtk_box_set_center_widget(a.native(), child.toWidget())
  	}
  }
docs: //gtk_box_bar_set_center_widget(GtkBox *box,GtkWidget *center_widget)