Starts a new drag event for this tree widget.  Overloading from the
        default QTreeWidget class to define a better pixmap option when
        dragging many items.
        
        :param      supportedActions | <QtCore.Qt.DragActions>

def startDrag( self, supportedActions ):
        """
        Starts a new drag event for this tree widget.  Overloading from the
        default QTreeWidget class to define a better pixmap option when
        dragging many items.
        
        :param      supportedActions | <QtCore.Qt.DragActions>
        """
        if ( not self.useDragPixmaps() ):
            return super(XTreeWidget, self).startDrag(supportedActions)
        
        filt  = lambda x: x.flags() & QtCore.Qt.ItemIsDragEnabled
        items = filter(filt, self.selectedItems())
        if ( not items ):
            return
        
        data = self.mimeData(items)
        if ( not data ):
            return
        
        if ( len(items) > 1 ):
            pixmap = self.dragMultiPixmap()
        else:
            pixmap = self.dragSinglePixmap()
        
        # create the drag event
        drag = QtGui.QDrag(self)
        drag.setMimeData(data)
        drag.setPixmap(pixmap)
        
        drag.exec_(supportedActions, QtCore.Qt.MoveAction)