/// <summary>
        /// Register html to flow document attribute value converter
        /// </summary>
        /// <param name="htmlAttName"></param>
        /// <param name="converter"></param>
        /// <param name="isReplaceOld"></param>
        public static void RegisterHtmlToFlowDocValueConverter(string htmlAttName, ValueConverter converter, bool isReplaceOld)
        {
            if (isReplaceOld)
                _HtmlToFlowDocValueConverter.Remove(htmlAttName);

            _HtmlToFlowDocValueConverter.Add(htmlAttName, converter);
        }
        /// <summary>
        /// Register flow document to html attribute value converter
        /// </summary>
        /// <param name="flowDocAttName"></param>
        /// <param name="converter"></param>
        /// <param name="isReplaceOld"></param>
        public static void RegisterFlowDocToHtmlValueConverter(string flowDocAttName, ValueConverter converter, bool isReplaceOld)
        {
            if (isReplaceOld)
                _FlowDocToHtmlValueConverter.Remove(flowDocAttName);

            _FlowDocToHtmlValueConverter.Add(flowDocAttName, converter);
        }