docs: public void addConnectionListener(KNXListener l)
  	{
  		if (l == null)
  			return;
  		synchronized (listeners) {
  			if (!listeners.contains(l)) {
  				listeners.add(l);
  				listenersCopy = new ArrayList(listeners);
  			}
  			else
  				logger.warn("event listener already registered");
  		}
  	}