title: Dynamically added items to nanogallery2 are not shown
body: <p>I am using nanogalley2 to show a galley on a web page. Items of this gallery are fetched from a server and added dynamically to the gallery. I add items based on the <a href="https://codepen.io/Kris-B/pen/wJKowg" rel="nofollow noreferrer">code snipped</a> provided by the official website. But my dynamically added items are not shown (in the cascading layout). I debug the code in the chrome developer tools and I see that items are added to the list of items of the gallery but I had no idea why these items are not shown.</p>
  
  <p>Here is my HTML code:</p>
  
  <pre><code>&lt;div id="my_nanogallery2"
      data-nanogallery2='{
          "itemsBaseURL" : "https://example.com",
          "thumbnailWidth" : 310,
          "thumbnailHeight" : "auto",
          "thumbnailDisplayTransition" : "slideUp",
          "thumbnailAlignment" : "center" 
      }'&gt;nanogallery2
      &lt;a href="my-first-image.jpg" data-ngthumb="my-first-image.jpg" data-ngdesc=""&gt;My First Image&lt;/a&gt;
  &lt;/div&gt;    
  &lt;button type="button" id="btn_add"&gt;Load more ...&lt;/button&gt;
  </code></pre>
  
  <p>and the JS code:</p>
  
  <pre><code>var options = {
      baseUrl : 'https://example.com',
      thumbnailWidth : 310,
      thumbnailHeight : 'auto',
  };
  jQuery('#btn_add').on('click', function() {
      $http('/api/rest/to/get/some/content')//
      .then(function(res){
          var newItems = res.data.items;
          newItems.forEach(function(item) {
              var instance = $("#my_nanogallery2").nanogallery2('instance');
              // create the new item
              // Parameters: instance, title, description, ID, albumID, kind, tags
              var newItem = NGY2Item.New(instance, item.title, '', item.id, '0', 'image', '');
              // define thumbnail -&gt; image displayed in the gallery
              var itemUrl = options.baseUrl + '/' + item.url;
              newItem.thumbSet(itemUrl, options.thumbnailWidth, options.thumbnailHeight); // w,h
              // define URL of the media to display in lightbox
              newItem.setMediaURL(itemUrl, 'img');
              newItem.addToGOM();
          });
          $("#my_nanogallery2").getElement().nanogallery2('resize');
      });
  });
  </code></pre>