title: PyQt QTableWidget signal emitted when selecting no rows
body: <p>I have a <code>QTableWidget</code> in <em>pyQt</em> with single row selection set.  I am connecting <code>itemSelectionChanged</code> to call my row selection function and take action on the selected row.  I would also like to detect when the user selects inside the <code>QTableWidget</code>, but selects the empty space (no row is selected), so that I can deselect any selected row.  Similar to how "windows explorer" works with file selections.  </p>
  
  <p>What signal is triggered when selecting the blank area inside a <code>QTableWidget</code>?  How can this be accomplished?</p>