TkAgg RadioWidget

I hope it is not something I have broken...

    > I have never looked at or tested the widgets before.

It looks like mouse event handling is being swallowed. It looks like I'm
not getting x-y cursor updates in the toolbar.

Looking at the CHANGELOG, I suspect

2006-03-17 Applied sophana's patch to support overlapping axes with
           toolbar navigation by toggling activation with the 'a' key.
           Closes sf patch 1432252 - JDH

And indeed, when I press 'a', the event handling comes back.

The odd thing is that Axes.__init__ is calling self.set_navigate(True)
(which is what 'a' does in backend_bases.FigureManagerBase.key_press.

So I'm confused, but at least we are on the scent.