constructor(callbackManager, context) {

    // create callback registry
    this.callbackManager = callbackManager;
    this.context = context;

  }
@param {CallbackManager} callbackManager - A callback manager that
    is informed about changed tiddlers and keeps track of the
    various tiddlers produced during the dialog process.
@param {Object} [context] - An optional *this*-reference to bind the
    callback of each called dialog to. Otherwise, the callback of
    each dialog has to be bound manually to the callback if required.