title: QSortFilterProxyModel: retrieve the item or index of the original model
body: <p>I happen to have a ComboBox, and a QSortFilterProxyModel attached to it that orders the items alphabetically.</p>
  
  <p>The Original model (QComboBox) is for example:</p>
  
  <p>"C",
  "A",
  "D",
  "B"</p>
  
  <p>The sorted model (QSortFilterProxyModel) becomes:</p>
  
  <p>"A",
  "B",
  "C",
  "D"</p>
  
  <p>If I now select "D" within the sorted comboBox, the signal QComboBox::currentIndexChanged(int) gives me index=3, but I would like to retrieve the index relative to the Original Model possibly, so index=2.</p>
  
  <p>On the other hand I would also like to "Original Model"->setCurrentIndex(idx). But if if I define idx=2, in the comboBox the highlighted row shows "C", as the view is sorted.</p>
  
  <p>How do I solve this?</p>