GtkImage peer rewrite

Sven de Marothy completely rewrote the image-handling sections of the GTK peers. This was one of the most convoluted areas in the peers and one of the buggiest. Now it is very clear and well-organized. Check out this screenshot:

The top is JDK 1.4, centre is Classpath 0.14 and bottom is Classpath HEAD with Sven’s patch. This work is awesome because it means we’ll have a solid foundation when we make the transition to using BufferedImages exclusively. Thanks Sven!