implement server and client player join/leave notifications

include/server/Server.h

@@ -8,6 +8,10 @@ namespace server {
 
 class Server {
   public:
+	/**
+	 * \brief Construct a server
+	 * \pre Two instances of Server should not share the same world
+	 */
 	Server(std::uint16_t a_Port, std::shared_ptr<Nz::EnttWorld> a_World);
 	~Server();