FCPPluginMessage: Add FIXME to have a separate field in messages which makes it very clear that they are a reply
- The distinction between non-reply and reply messages is very important for the new API because FCPPluginClient.sendSynchronous() *needs* a reply so it can determine when to return. So it should be ensured that custom client implementations don't break detection of reply messages. This would be very likely if we only used "Success" as an implicity indicator of replies.