Effects > Natural > Fog: total overhaul

Whew, another Nature filter rewrite down.  This function now provides a
full-blown fractal noise generator, using Perlin Noise as the base.
This is (presumably?) the same "Cloud" function used by software like
Photoshop and GIMP, and if I ever add a "Render" set of filters to PD,
this will be an easy reference for our own cloud function.

In the meantime, this dialog now allows the user to overlay a cloud-like
effect over an image, with controls for fog scale, contrast, density,
and quality.  (Quality controls the number of iterations used to
calculate the fractal noise; more = looks better = slower.)  This yields
a very realistic effect, IMO, and certainly much better than our
horrible old one-click solution.
Rename "Burn" filter to "Ignite"

Burn is potentially confusing, owing to the conventional Dodge/Burn
mechanic.