implement dragging

def on_mouse(self, event):
        '''
        implement dragging
        '''
        # print('on_mouse')
        if not event.Dragging():
            self._dragPos = None
            return
        # self.CaptureMouse()
        if not self._dragPos:
            self._dragPos = event.GetPosition()
        else:
            pos = event.GetPosition()
            displacement = self._dragPos - pos
            self.SetPosition( self.GetPosition() - displacement )