FredPluginFCPMessageHandler.FCPPluginMessage: Add asserts() to constructor
FredPluginFCPMessageHandler.FCPPluginMessage: Add constructors for outgoing messages; Encapsulate in factory functions
- This will allow server and client to construct outgoing messages to pass them to the FCPPluginClient send() functions. (Following commits will adapt the send functions)
- Also greatly improves the JavaDoc