This was my initial choice, but the more I think about it, the less I am
sure this was the right choice. There was some common code (as I was
making a common composite bucket fill once the line art was generated),
but there is also a lot of different code and the functions were filled
of exception when we were doing a line art fill. Also though there is a
bit of color works (the way we decide whether a pixel is part of a
stroke or not, though currently this is basic grayscale threshold), this
is really not the same as other criterions. In particular this was made
obvious on the Select by Color tool where the line art criterion was
completely meaningless and would have had to be opted-out!

This commit split a bit the code. Instead of finding the line art in the
criterion list, I add a third choice to the "Fill whole selection"/"Fill
similar colors" radio. In turn I create a new GimpBucketFillArea type
with the 3 choices, and remove line ...