First cut on a QtAgg backend

We've also been working on a Qt front end. It's been going

    > slow because we've only been able to spend about 1 oerson-day
    > per week on it. But, starting this week I have someone full
    > time on it. He's gotten a basic widget working using the
    > same organization that the GTK code uses. Sigve's code has a
    > lot more functionality in the tool bars than ours right now.

Great -- it looks like there will be a good opportunity to merge the
best features from both backends -- two functional qt backends is
definitely an improvement over what we had a week ago! Particularly
useful might be to retain the current implementation which uses string
methods to transfer the agg image to the qt canvas as a fallback for
your extension code method. That way, people who have trouble getting
the extension code version working can always fallback to something
slower that still works.

    > The guy working on it doesn't think it would take more than a
    > day or two to incorporate Sigve's code for the tool bar
    > handling into our code. We're close enough to finishing that
    > it probably doesn't make a lot of sense to try and set up a
    > collaboration right now but I'm open to that possibility if
    > that's what people want. All I really interested in is
    > seeing the QtAgg front end in matplotlib as a supported
    > component (which we'd be happy to help maintain) so whatever
    > gets us there the quickest sounds good to me.

Whatever works for you and Sigve works for me. My guess is that it
will be easy to cross-pollinate from one implementation to the other.