title: Clear selection when clicking on blank area of Item View
body: <p>I made a tree structure with <code>QTreeWidget</code>, and it works well. But I have a problem with that. Commonly, with a tree structure, if I want to deselect all, I click on a blank area of the tree-widget. But <code>QTreeWidget</code> does not seem to support that (or I can't find out how).</p>
  
  <p>Could sub-classing or something else solve the problem? Determining a click on a blank area seems the key to the solution, but I can't find a signal or anything.</p>