callback ideas (artist.Artist, mostly)

Would others find it useful to have most/all of the artist.Artist
subclasses updated to use the new (weak-ref) cbook.CallbookRegistry

I'm working on a tool to tweak matplotlib figure styles/colors, etc,
and I find it very useful to selectively enable "auto-updating" in
my own toolkit such that the figure is updated on each change that
I make. Although I'm able to do this in my own toolkit's wrapper
layer, I would rather do it within matplotlib itself.

My thoughts are that at the least, I'd want parents to be able to
subscribe to signals from children, and possibly vice-versa. Then
at the top, have the Figure optionally add it's draw() method as
a callback to it's child axes/whatever instances.

I'll probably start working on some patches for own use anyway,
but if there's interest, I'll keep them polished/tested and
submit them for review.