title: QVariant to QIcon/QPixmap/QImage
body: <p>I want to extract a QIcon I've stored in one of a QTreeWidget's columns, as Qt::DecorationRole.</p>
  
  <pre><code>QTreeWidgetItem *item = ui-&gt;treeWidget-&gt;topLevelItem(index);
  const QIcon &amp;icon = item-&gt;data(0, Qt::DecorationRole)._howToConvert_();
  </code></pre>
  
  <p>However, I can only get the data as QVariant, and I could not find a function to convert from a QVariant to QIcon. Is it possible to do it?</p>