code: func (a *ActionBar) SetCenterWidget(child IWidget) {
  	if child == nil {
  		C.gtk_action_bar_set_center_widget(a.native(), nil)
  	} else {
  		C.gtk_action_bar_set_center_widget(a.native(), child.toWidget())
  	}
  }
docs: //gtk_action_bar_set_center_widget(GtkActionBar *action_bar,GtkWidget *center_widget)