onSlideChange_(slide) {
    if (this.currentSlide_) {
      this.currentSlide_.disable();
    }

    this.currentSlide_ = slide;
    this.currentSlideI_ = slide.i;
    this.currentSlide_.enable();
    this.isMoving = false;

    DOM.fireEvent(this.el, 'ws:slide-change', {
      slides: this.maxSlide_,
      currentSlide0: this.currentSlideI_,
      currentSlide: this.currentSlideI_ + 1
    });
  }
Whenever a slide is changed, this function gets called. It updates the
references to the current slide, disables the moving flag and fires
a custom event.
@param {Slide} slide The slide we're transitioning to.
@fires WebSlide#ws:slide-change
@private