FCPPluginClientTracker: Implement registerClient() and getClient()
- Inspired by the WOT class ClientTrackerDaemon: https://github.com/freenet/plugin-WebOfTrust/blob/e5e9d1df3e62a92964807dafe13cecc0749078e7/src/plugins/WebOfTrust/ui/fcp/FCPInterface.java#L116