add widgetAdded and widgetRemoved

/** * \brief The @p widget was added to this window. * \param widget the widget that got added * \since 6.0 */ void widgetAdded(QWidget *widget);

/**
 * \brief The @p widget was removed from this window.
 * \param widget the widget that got removed
 * \since 6.0
 */
void widgetRemoved(QWidget *widget);