getElementIndex(node) {
        let index = false;
        _.each(this.elements, (el, i) => {
            if (_.has(el, 'node') && el.node == node) {
                index = i;
                return true; // exit loop
            }
        });

        return index;
    }
Return the index
of the specified node,
this node is for example an image, link, etc.
that when clicked it opens the lightbox
its position in the elements array can change
when using insertSlide or removeSlide so we
need to find it in the elements list

@param {node} node
@return bool|int