Effects > Find Edges: overhaul dialog and underlying code

One of the recurring requests I get for PD is more/better artistic
effects, specifically ones that recreate a "hand-painted" sort of look.
There are a lot of ways to create such filters, but many rely on
edge-processing algorithms to find image edges, which the filters than
further process to mimic a "hand-painted" look in varying styles.

To that end, I have overhauled PD's edge detection code (and
corresponding dialog) to make it more amenable to internal use by other
functions.  Directionality is now explicitly supported, with capable
filters updated to process edge detection in horizontal, vertical, or
both directions simultaneously.  PD's (awesome!) layer compositor allows
us to blend results from multiple blend directions quickly and
accurately, and of course all previous edge-detection features are
unchanged by the overhaul.

This new structure will also make it much easier to add more advanced
edge detectors (Canny, etc) in the future, should I feel so inclined.