title: QTableView - sort headers
body: <p>Is it possible to sort the headers of a QTableView?
  <img src="https://i.stack.imgur.com/YGJ6x.png" alt="enter image description here"></p>
  
  <p>I'd like to sort the horizontal and vertical headers. QTableView only offers functionality to sort a column's contents. 
  Is there a way to sort the view or do I have to sort the underlying datastructures?</p>