message: Change callbacks of Event from private to protected
diff: @@ -878,7 +878,7 @@ export class SurveyElement extends Base implements ISurveyElement {
  }
  export class Event<T extends Function, Options> {
  - private callbacks: Array<T>;
  + protected callbacks: Array<T>;
  public get isEmpty(): boolean {
  return this.callbacks == null || this.callbacks.length == 0;
  }
  