Resize bug in tkagg backend

I have a suggestion for a feature in the ‘resize window’ discussion.

We really need some kind of callback when a window is resized manually
in interactive mode. That is, when somebody points his mouse to the
corner of a window and drags it to a bigger size. When this happens,
and the plot in the window is supposed to be drawn to scale (so that
circles look like circles), the size of the plot needs to be managed.
That is not difficult to do, as long as their is a signal given that
the window was resized.