PanTool does not restore window mouse owner after panning

[mdsmarte]

Problem Description

The PanTool sets the window mouse owner to be itself when panning starts (_start_pan):

https://github.com/enthought/chaco/blob/192675d6cb579df56458b87988026d895e7a7a84/chaco/tools/pan_tool.py#L257-L259

When panning stops (_end_pan), it sets the window mouse owner to None:

https://github.com/enthought/chaco/blob/192675d6cb579df56458b87988026d895e7a7a84/chaco/tools/pan_tool.py#L269-L270

Expected behavior:

I would have expected _end_pan to revert the mouse owner back to the original owner before panning started, rather than changing it to None.