title: Drag and drop support when subclassing from QAbstractItemModel
body: <p>I subclassed from <code>QAbstractItemModel</code>, put model into <code>QTreeView</code>, everything is fine.</p>
  
  <p>Now I want to support drag and drop, so it will be possible change item position with mouse.<br>
  I set to <code>QTreeView</code> <code>dragDrop</code> mode to <code>DragDrop</code>, <code>defDropAction</code> <code>MoveAction</code>, <code>drag</code> enabled to true. I can drag item, it appears near mouse, but on putting nothing happens.</p>
  
  <p>What methods from <code>QAbstractItemModel</code> I need to realize that my model will support drag and drop?</p>