Add onProgressText event

[andrewtelnov]

Allow developers to set any text into progress bar.

  /**
   * Use this event to change the progress text in code.
   * <br/> `sender` - the survey object that fires the event.
   * <br/> `options.text` - a progress text, that SurveyJS will render in progress bar.
   * <br/> `options.questionCount` - a number of questions that have input(s). We do not count html or expression questions
   * <br/> `options.answeredQuestionCount` - a number of questions that have input(s) and an user has answered.
   *  @see progressBarType
   */
  public onProgressText: Event<
    (sender: SurveyModel, options: any) => any,
    any
  > = new Event<(sender: SurveyModel, options: any) => any, any>();

Example of showing % of answered questions:

  survey.onProgressText.add((sender: SurveyModel, options: any) => {
    questionCount = options.questionCount;
    answeredQuestionCount = options.answeredQuestionCount;
    options.text =
      "Answered: " +
      (100 * options.answeredQuestionCount) / options.questionCount +
      "%";
  });