FCPInterface.sendUnsubscribedMessage(): Adapt to fred plugin-fcp-rewrite

- The new FCPPluginClient.send() needs a SendDirection and throws
IOException instead of PluginNotFoundException (same meaning: Connection
closed).
FCPInterface.handleUnsubscribe(): Fix return type

- Make it match the backend other handleUnsubscribe()
FCPInterface: Adapt to FCPPluginMessage.parameters rename to params

- This was done in fred plugin-fcp-rewrite commit
3980e1c9bc015f8af3518f3ac738a9d9801947ef