Extensibility pt.1: Server-side WebSocket events.

Introduce `WebsocketTextMessageListener` event.

Plugins can listen to incoming text messages (server-side):
{{{
import de.deepamehta.plugins.websockets.event.WebsocketTextMessageListener;

public class WebSocketsExamplePlugin extends PluginActivator implements WebsocketTextMessageListener {

    @Override
    public void websocketTextMessage(String message) {
        ...
    }
}
}}}

More events possibly to come.