Implement UiMessageHandler tailored to use JsonCodecs

As a UI View developer, I would like my subclass of UiMessageHandler to be able to use the JsonCodec classes for creating JSON descriptions of objects, thus providing helper methods in generating payloads to send up to the client.

Implementation Notes:

• UiMessageHandler should be augmented to create a private inner class that implements CodecContext, to facilitate the use of JsonCodecs.