... in GimpBucketFillOptions for the line art algorithm.

Inside GimpLineArt, there are still 2 properties, but we don't show them
anymore in the Bucket Fill tool options. One of the main reason is
probably that it's hard to differentiate their usage. One is to close
with curved lines, the other with straight segments. Yet we don't
actually have any control on one or the other. All one knows is that you
can have "holes" in your drawing of a given size and you want them
close-like for filling. Only reason I can see to have 2 types of closure
is whether you'd want to totally disable one type of closure (then you
set it to 0). But this is a very limited reason for making the options
less understandable overall, IMO.
So for the time being, let's show up only a single option which sets
both properties in GimpLineArt. As patdavid says "it makes sense as a
first pass".

Also rename the option to shorter/simpler "Maximum gap length". Thanks
to patdavid and pippin for helping on figuring out this better label!

Finally I am bumping the default for the gaps to 100px. The original
values were ok for the basic small images used in demos, but not for
real life image where it was always too short (even 100px may still be
too short actually, but much better than the 20 and 60px from before!).