The interaction is fully restored, it's also ported to the new halt()
and commit() API of tools and semi-properly GEGLized: the gradient map
is now constructed on the fly using a GimpTileHandlerValidate
subclass.

The only problem is that it doesn't find any edges in the image and is
totally useless. Pushing anyway to put an end to the bitrot, any help
with debugging is greatly appreciated...