DragTool mouse down event handling

When trying to use DragTool, dragging will not occur unless I also implement my own normal_left_down function:

``` python
def normal_left_down(self, event):
event.handled = True
```

Unless I implement this method, the `left_down` attribute of the subsequent mouse move events will be False, and dragging will not be handled correctly by the DragTool. Is it correct that I have to implement this extra handling to make it work? And what is the dependency of the `MouseEvent.left_down` attribute to previously handled events?

Also the DragTool does not use the modifier keys for anything.