Effects > Find edges: add Roberts Cross algorithm

http://en.wikipedia.org/wiki/Roberts_cross
Effects > Enhance Edges: top-to-bottom overhaul

Edge enhancement is an extremely useful technique, so it was time to
give the function proper treatment.  (In many cases, superior sharpening
results can be obtained via this function than via a naive Sharpen
approach.)

A dialog similar to Find Edges is now provided, which allows the user to
select any edge detection algorithm as the basis for edge enhancement.
Directionality and variable strength is also supported, and once again,
the pdComposite class has proven a lifesaver.  (It handles all image
merging, which happens at least once and possibly more in this function
depending on the edge detection technique used.)