IMessage & SimpleMessage (Relaxed Message Sending p1)

[philipwhiuk]

Drawn from a need to send out messages previously stored without use of a DataDictionary.
This occurs for two reasons:

• QFJ only validates inbound messages not outbound messages. You can send messages that don't match the dictionary
• It's possible (fairly common) to disable use of a Dictionary (UseDataDictionary=N).

NB:

This PR doesn't implement all of it - that needs changes in the App code to use IMessage instead of Message.