app: handle migration on OSX of GIMP 2.8.2's config folder.

GIMP 2.8.2's config was "~/Library/Gimp/x.y", before it got moved to
"~/Library/Application Support/x.y" and after being saved in the home
like other UNIXes. The migration code will now check all 3 places in the
right order on OSX.
Thanks to Simone Karin Lehmann for the original proposition.