FCPPluginReply: Add FIXME to rename it to FCPPluginServerMessage

- The new FCPPluginClient API allows the server to send
NON-reply messages to the client, i.e. messages triggered solely by
events at the server, not by a message from the client. So "Reply"
should not be in the class name anymore.
- Also, FCPPluginMessage will likely be renamed to
FCPPluginClientMessage, so this class here should have a matching name.