Allow copying custom text instead of QChar::ObjectReplacementCharacter

If you add QChar::ObjectReplacementCharacters coupled with your custom QTextCharFormat, it's not possible to replace those ObjectReplacementCharacters with meaningful text, even if that would be the right way to do (when trying to implement a widget to display chat history, with emoticons, which are replaced from '' and the like text patterns).

But if you have already registered your own QTextObjectInterface handler for custom QTextCharFormat, and probably it could be extended to return meaningful HTML markup instead of simple ObjectReplacementCharacter.