Displays the header menu for this tree widget.
        
        :param      pos | <QtCore.QPoint> || None

def showHeaderMenu( self, pos):
        """
        Displays the header menu for this tree widget.
        
        :param      pos | <QtCore.QPoint> || None
        """
        header = self.header()
        index  = header.logicalIndexAt(pos)
        self._headerIndex = index
        
        # show a pre-set menu
        if self._headerMenu:
            menu = self._headerMenu
        else:
            menu = self.createHeaderMenu(index)
        
        # determine the point to show the menu from
        if pos is not None:
            point = header.mapToGlobal(pos)
        else:
            point = QtGui.QCursor.pos()
        
        self.headerMenuAboutToShow.emit(menu, index)
        menu.exec_(point)