AppImage library content still lacking

While it factually works for the most popular Desktop Linux distributions, there are still problems the automatic tests pick up.

The included libxml2 references a libicudata version that is only rarely available. It's far too large to be included, so we need to build our own libxml without icu.

We also don't have all dependencies of SDL-Image included.

The popular distributions have both libxml and SDL-Image included and we use the system version.