/// <summary>
        /// HeaderProperty property changed handler.
        /// </summary>
        /// <param name="d">HeaderedContentControl whose Header property is changed.</param>
        /// <param name="e">DependencyPropertyChangedEventArgs, which contains the old and new value.</param>
        private static void OnHeaderPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            HeaderedContentControl ctrl = (HeaderedContentControl)d;

            ctrl.OnHeaderChanged(e.OldValue, e.NewValue);
        }
        /// <summary>
        /// HeaderTemplateProperty property changed handler.
        /// </summary>
        /// <param name="d">HeaderedContentControl whose HeaderTemplate property is changed.</param>
        /// <param name="e">DependencyPropertyChangedEventArgs, which contains the old and new value.</param>
        private static void OnHeaderTemplatePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            HeaderedContentControl ctrl = (HeaderedContentControl)d;

            ctrl.OnHeaderTemplateChanged((DataTemplate)e.OldValue, (DataTemplate)e.NewValue);
        }