title: QTableView: dataChanged event clears cell being edited
body: <p>Working with a <code>QTableView</code> and <code>QAbstractTableModel</code> - when the model emits a <code>dataChanged</code> event for the cell being edited,  the string the user has typed in the cell (but not pressed enter to 'commit' the edit) is erased.</p>
  
  <p><em>Example</em>: Click a cell, type '123', cell is still in edit mode waiting for more text, <code>dataChanged</code> is emitted and the '123' is erased, leaving an empty cell in edit mode.</p>
  
  <p>Does anyone know how to stop this behaviour, or how the model can detect when the cell is being edited to prevent <code>dataChanged</code> events being raised for that cell?</p>