Gimp# needs Gimp 2.2-pre

I just found out that Gimp# is dependant on Gimp 2.2-pre (or 2.1.x). Since 2.2 is probably going to be released before the end of this year, I'm not going to try to backport Gimp# to 2.0. This also means that I now can make use of the new preview widget.