title: Add widgets to QFileDialog
body: <p>I need to add a widget (<code>QTableWidget</code>) into <code>QFileDialog</code>'s layout. I know that it is <code>QGridLayout</code> with sizes <code>(3,4)</code>. The table must be in 3-rd row and span all columns. </p>
  
  <pre><code>QTableWidget* tableWidget = new QTableWidget(this);
  QGridLayout *layout = static_cast&lt;QGridLayout*&gt;(QFileDialog::layout());
  layout-&gt;addWidget(tableWidget, 2, 0, 1, 4);
  </code></pre>
  
  <p>With this code the original 3-rd row which contains lineEdit and save/open pushButton disappears. How can I add widgets between already existing widgets of <code>QGridLayout</code> so that original widgets remain in the layout. </p>