Added TextValue property to expose the xaml string value for a property.

If there is another way for me to get the value for a property where the type converter fails please let me know, as it might make this fix unnecessary.

My first thought was to add a property with the signature "object PrimitiveValue", but is it even remotely possible that there will be other primitives than string/text represented?
If this feels like a better option let me know and I'll make another pull request.