build: add an initial flatpak-builder manifest.
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.