games: Add sunless-skies.

This adds sunless-skies from gog. It's also available on humble bundle, so if any of you have it there, please let me know, so we can try and have a unified definition for both origins :). It works well but is quite big: 1.2 GB for the "sources" and 3.8 GB installed. You'll need at least 3 or 4 GB of RAM to download the sources to the store, but that's what's needed to run the game anyway. I guess it's not a lot bigger than factorio for instance, but it's probably approaching the limit of what we should accept as a proper package. What do you think?

This package gave me a lot of troubles: black screen at startup, no useful console message or logs. Strace was very helpful with this one ^^. I have two other games, but one of them is not yet working (black screen after the startup menu, logs seem to suggest it tries to write a file to the store) and the other is too big to be added to my store (5.7 GB of "sources") and I'll need to find another way. That other game works very well, but I could only patchelf it outside of any guix recipe.