Feature: General edge detection filter that works with filter layers.

This adds a general edge detection filter that encompasses all the features
of the existing edge detection filter in one filter.

On top of that, it has mechanisms to grow the kernel size dynamically, like
you can do with the gaussian and regular blur, three different kernel formulas
and the ability to write the result to the alpha channel, so that this filter
can be used to make cool fringe and watercolory effects.

CCMAIL:kimageshop@kde.org
Differential Revision: https://phabricator.kde.org/D6556