Bug 553855 - Bucket fill colour is always blended with filled pixel colour...

...when threshold > 0

Add an "Antialias" toggle to the bucket fill options and set it on the
GimpFillOptions. In gimp_drawable_bucket_fill(), pass it to
gimp_pickable_contiguous_region_by_seed() instead of always defaulting
to TRUE.

The position of the toggle and its huge tooltip may need some
adjustment.