title: JavaFX8 TreeTableView multiple root items
body: <pre><code>TreeItem&lt;FileModel&gt; root = new TreeItem&lt;FileModel&gt;(playlist);
  treeTblViewFiles.setRoot(root);
  treeTblViewFiles.setShowRoot(true);
  </code></pre>
  
  <p>The code above sets one root item, but I need to have several so called root items with expandable list of child items. How do I do that?<br/>
  Thanks in advance.</p>