Push Wesnoth achievements to Steam as well

Oftentimes someone on Discord will ask for Steam achievements or some other sort of feature that requires integration with the Steamworks API, only to be reminded that Steam is proprietary, and thus can't be linked against by GPL-ed games. When this comes up, often Iris (or someone) will bring up this "steamshim" project by @icculus that apparently works around it, so I'm opening this bug to track integration of it, so we have a place to refer people to when it comes up again: https://github.com/icculus/steamshim