Defringe tuning

1) The strength slider was removed because:
 increasing it: does not do much more than as if decreasing threshold
 decreasing it: very similar to using blending or increasing threshold
2) Some parameter tuning and small corner case error corrections.
3) There was also an issue found with the filter radii when using low values, at the same time the artifact filter can now be incorporated in the normal if-condition, reducing code that became redundant by the radius changes
4) The picture of the manual was changed to be without strength slider (the manual already misses the strength slider explanation, as it should be now)

We discussed to remove the strength slider on IRC before, you might want to review the other changes, but I don't have any near term plans for this anymore, except if bugfixing is required.