This diff changes the way GUI messages created within the NetClient are obtained by the GUI.
rP7653 introduced the Engine.PollNetworkClient() JSInterface method which is called onTick from GUI pages listening to the NetClient.
This however means that if you have 100 FPS and 1 network message per second, you will get 100 function calls where only one of them is relevant.
This means that this is an unscalable pattern, if every event-subscription system would check onTick, the code would become very ugly on top of performing badly.
Hence this diff changes the way of propagation to have C++ call the JS functions if and only if there is a NetClient message pending.