First cut on a QtAgg backend

Hi, I have written a basic QtAgg backend for

    > Matplot-lib. It is only tested on Windows, so some more
    > testing should be done, but it is my hope that it can some
    > day be included in the matplotlib distribution. I will test
    > on linux when I get the time, but all the code is python so
    > it should probably work ok.

Indeed -- it's a testament to QT and python that the untested code
appears to run the same on linux (per Steve's report) and OS X, which
I tested today. Good work!

    > See for the code.

    > Most examples run ok, but the does not
    > work yet. Not all examples have been tested, just some
    > random double-clicking on the one that look interesting.

I think the problem here was that you had

    def keyPressEvent(self, ev):
    def keyReleaseEvent(self, ev):

where you meant

    def keyPressEvent(self, ev):
    def keyReleaseEvent(self, ev):

I made these changes in CVS, and the keypress_demo indeed works.

    > A lot of the comments from the has not
    > been removed or fixed. The code is also somewhat rough
    > around the edges, but it works.

    > Is there any unittest-suite it is possible to run the
    > backend against?

The standard test suite is examples/backend_driver. You just need to
set the "backends" list to the list of backends you want to test. Eg

backends = ['QtAgg']

    > Feedback is welcome.

I agree with most of the comments Steve made.

  * I think factoring the class into a module that handles the qt
    widgets, and have a small module that does qtagg would be a good
    thing. Steve would likely do a qtcairo backend if you did this...

  * The only explanation I can think of for why the colors are wrong
    is that qt has a different byte order for images than you are
    getting from agg (is could it be bgra instead of rgba, or that
    something is wrong with the endianess...) If you need a different
    byte ordering /pixel format, I can provide the required method in
    the agg backend. I note that the saved figures *do* have the
    right color scheme, which supports this idea.
  * The control key modifier is apparently not recognized -- this
    provides aspect ratio preserving zoom. I can't test any of the
    right click drag functionality right now, actually, since I'm on a
    mac w/o an external mouse plugged in **&$$ macs!