Improve singleplayer game support

[ike709]

Right now moony is doing some shenanigans to list Robust Sand, a singleplayer game, on the server list. And when it's not listed you can't play it, even if you've already connected before and therefore downloaded it.

It would be nice if the launcher had a "Singleplayer" tab for the singleplayer games, and a saner way to list singleplayer games on it. Or at least a way to easily launch singleplayer games you've downloaded before.

This would also give you something to do if your internet is down.