title: Have multi-column QTreeWidget root items
body: <p>I am using a QTreeWidget to display items in categories. Items will use several columns, but I want categories (ie. root items) to use the full width of the widget.</p>
  
  <p>How can I do that?</p>
  
  <p>And a piece of my code:</p>
  
  <pre><code>class BugsList(QtGui.QDialog):
      def __init__(self, parent, reports):
          super(BugsList, self).__init__(parent) # Call QDialog constructor
          self._tree = QtGui.QTreeWidget(self)
          self._tree.setColumnCount(NUMBER_OF_COLUMNS)
          # ...
          for category, bugs in reports:
              category_widget = QtGui.QTreeWidgetItem(self._tree, [category])
              # ...
  </code></pre>
  
  <p>Here is a screenshot of the current state of my application:
  <img src="https://i.stack.imgur.com/ouUTr.png" alt="screenshot"></p>