Tools can now have an in-image context menu which you get when right-
clicking inside the image editing area. Our first use of this is to
provide the ability to move layers to the back/front. :^)
The main editing widget is now the new ImageEditor widget, which works
on an Image object, which internally has a stack of Layer objects.
Layers are composited back-to-front when painting the Image inside an
ImageEditor.
