Implement prefiltering algorithms for Lazy Fill Tool

Now colorize mask has several options:

* "Edge detection (px)" --- if activated, the mask tries to find edges of
  the filled areas in the source image. This option should be activated
  **only** if the source image has vast areas filled with (almost) solid
  color.

  Important: in case you image is a mixture of lines and solid blobs, make
             sure that the "size" of edge detection filter is set to the
             size of the lines in your image. That will give you the best
             results.

* "Gap close hint (px)" is the size of the gaps the filter will try to
  "close" before running the flood filling. This value should be about
  the size of the maximum unclosed gap in your image.

* "Clean up (%)" is a special mode of the mask, when it tries to remove
  some of your key strokes if they go beyond the "closed" areas. When 0%,
  clean up phase is skipped, when 100% it tries to remove as much
  "uncertain" details as possible.

CC:kimageshop@kde.org