Add Slic3r, a 3D printer tool

@the-kenny: I hope you don't mind that I post this pull request now that you have posted yours :-) I am just eager to get the slic3r branch merged. It feels like I've had it in my tree forever... well, according to git, since 28th of april.

I have
 - bumped Slic3r to the latest 0.9.10b release (which according to the author works fine, even if the tests don't work[1])
 - added desktop item
 - cleaned up my branch so it's easy to review

Slic3r has issues with perl 5.16 (nixpkgs default), so I use perl 5.14. I have also made a commit that enables building of some modules against perl 5.10 (I tried to use perl 5.10 with slic3r for a while).

[1] https://github.com/alexrj/Slic3r/issues/1303