toc improvements

This adds a check that a toc entry actually exists before adding it, so that if some bit is missing on a particular platform (e.g., GEM on the Mac), the corresponding toc entry will be hidden. I also added the DISIS and Gem entries back to the toc, so that they will be shown if available on the target platform.

Yes, this uses fs.accessSync, but I didn't notice any perceivable delay because of this. We're not scanning a ton of entries here, so it should be ok.