Based on Alexander Larsson's original nightly GIMP flatpak. I updated
most dependencies, changed some options, and added some dependencies
whose versions were now too low in the runtime (libpng and lcms2).
I also use the new "--filesystem=xdg-config/GIMP" option, made upon my
request, which means we will want to use a recent flatpak.
Many options are still missing but that's a start with a working
flatpak manifest.