title: Qt QModelIndex::sibling return parent QModelIndex
body: <p>Having a QAbstractItemModel <code>grModel</code> and calling:</p>
  
  <pre><code>QModelIndex grSrcPrtIdx = grModel.index( 1, 0 );
  QModelIndex grSrcIdx1   = grSrcPrtIdx.sibling( 1, 0 );
  QModelIndex grSrcIdx2   = grModel.index( 1, 0, grSrcPrtIdx )  
  </code></pre>
  
  <p>Results in: <code>grSrcPrtIdx == grSrcIdx1</code> with <code>grSrcPrtIdx</code> containing the data expected for <code>grSrcPrtIdx</code> while <code>grSrcIdx2</code> contains the expected data for the sibling. My expectation is that <code>grSrcIdx1 == grSrcIdx2</code>.</p>
  
  <p>It seems that the sibling function provides a defaut implementation (or something similar?). </p>
  
  <p>What might be the cause that the sibiling function does not deliver the siblings <code>QModelIndex</code>? Did I miss to implement a function of the <code>QAbstractItemModel</code>? </p>
  
  <p>Qt version used is Qt 5.10.0 (MinGw).</p>