_initNavigation (event, eventData) {
    if (arguments.length === 2) {
      this._activeElement = eventData.detail.modal;
    } else {
      this._activeElement = event.detail.modal;
    }

    if (this._activeElement.querySelectorAll('.css-modal_detail').length === 0) {
      return;
    }

    // Bind events on the current modal element and set the currentItem to 0
    if (!this._activeElement._galleryEventsBound) {
      this._bindEvents(this._activeElement);
      this._activeElement._galleryEventsBound = true;
      this._activeElement._currentItem = 0;
    }

    // If we have an incoming index, override the current item
    if ('index' in this._activeElement) {
      this._activeElement._currentItem = this._activeElement.index;
    }

    this._detailView = this._getDetailView(this._activeElement);
    this._currentItem = this._activeElement._currentItem;
    this._items = this._readContent(this._activeElement);

    this.setActiveItem(this._currentItem);
  }
Registers the listerns on the previous / next buttons to enable gallery
navigation
@return {void}