Gimp 3.0.0. rc1 AppImage

[Tas_mania]

Thanks ofnuts, I think the DevOps method works well with appimages because the image is self-contained. Just unpack, add or remove files, re-pack and test. 

If anyone wants to do testing on the official Gimp 3 appimage check here:
build/linux: Make AppImage run in (possibly) every distro

The appimage proposal is here:
https://gitlab.gnome.org/GNOME/gimp/-/issues/7661