Added OpDialogue.messageWidget() method.

This can be used to get access to the widget used to display messages from the op.
Added MessageWidget.forwardingMessageHandler() method.

This allows additional message handlers to be registered, so that messages can be passed on in addition to being captured and displayed by the widget.
Removed deprecated MessageWidget method usage from OpDialogue.

This also means that the result and error messages we append will make it to the MessageWidget.forwardingMessageHandler(), whereas with the deprecated methods they would not.