constructor(appId = '', isGreedy = false, onclose = '', debug = false) {
    let cmd = {'init': {}};
    if (appId != '') cmd['init'].appid = appId;
    if (onclose != '') cmd['init'].onclose = onclose;
    if (isGreedy) cmd['init'].greedy = true;
    if (debug) cmd['init'].debug = true;
    super(cmd, null);
  }
@constructor
@param {string}  appId - a unique id for app
@param {boolean} isGreedy - will it take over screen
@param {string}  oncloseBehavior - can be 'wipe', 'hide', 'none'