Delete interface FredPluginFCPClient:
- Fix users to use the replacement ClientSideFCPMessageHandler. Done with Eclipse auto-renaming and then manually reviewed: In some instances, it made sense to explicitely reference it as "FredPluginFCPMessageHandler.ClientSideFCPMessageHandler" instead of only "ClientSideFCPMessageHandler" because we want plugins to use interfaces which start with "FredPlugin*". So in the classes relevant to them, we should stress that by referencing the extended name.
- Removes the FIXME demanding the renaming+deletion of the old interface at FredPluginFCPMessageHandler
- Indentation/line wrapping will be in the following commit.