Make it possible to save/load custom handler objects from a QAbstractTextDocumentLayout

Currently if a custom handler object is registered for a QAbstractTextDocumentLayout and an ObjectReplacementCharacter is inserted into the document, there is no trace of this when the document is converted to HTML or plain text. It would be nice if a virtual function in the handler was called when the document is saved or loaded so that it would be possible to store the object data in the file.