graphics/gimp3-app: add GIMP 3 to the ports tree (+)

Add GIMP 3.0.2 as separate port until plugins (including GutenPring) will not be ported to new API

Ancient MNG support is disabled and marked as BROKEN, our graphics/libmng port is borked by design

Release notes: https://www.gimp.org/news/2025/03/16/gimp-3-0-released/ \

		https://www.gimp.org/news/2025/03/23/gimp-3-0-2-released/