Add support for ToUnicode CMap stream.

A side effect of not supporting it is the following: exporting PDF document which, for example, contains German umlauts, to plain text, leads to wrong characters in the resulting text.

As possible workaround, the PdfFormatProvider instance of RadPdfViewer can be used to import the document. For example:

FormatProviderSettings settings = new FormatProviderSettings(ReadingMode.OnDemand);
PdfFormatProvider pdfViewerFormatProvider = new PdfFormatProvider(stream, settings);
 
RadFixedDocument document = pdfViewerFormatProvider.Import();
 
TextFormatProvider textFormatProvider = new TextFormatProvider();
String text = textFormatProvider.Export(document);