FCPConnectionHandler.getPluginClient(): Pass the plugin to FCPPluginClient constructor
- The previous commit changed the constructor to demand it
- FCPPluginMessage will use this function. It shouldn't deal with the low level code of obtaining the plugin so this is the right place to do this. Also, FCPPluginClient object will be persisted across the processing of multiple FCPPluginMessages, so FCPPluginMessage shouldn't query the plugin each time. The fact that the FCPPluginClient stores it as a WeakReference internally guarantees that we don't force-keep it in memory. Also, its send-functions will check whether the reference is valid, so FCPPluginMessage will be able to indicate failure if the plugin has been unloaded meanwhile