new embedding_in_wx3 example

Hi All,

I've added a new example, embedding_in_wx3, to CVS. From the docstring:

This is yet another example of using matplotlib with wx. Hopefully
this is pretty full-featured:

  - both matplotlib toolbar and WX buttons manipulate plot
  - full wxApp framework, including widget interaction
  - XRC (XML wxWidgets resource) file to create GUI (made with XRCed)

This was derived from embedding_in_wx and dynamic_image_wxagg.

Thanks to matplotlib and wx teams for creating such great software!

John+Perry+whoever: the only matplotlib bug I found this time is worked around by the line

rcParams['image.origin'] = 'upper' # 'lower': nav toolbar problem -ADS

We should fix that at some point.