Gameplay selection

This is meant in part as a discussion topic for an implementation of some parts of this forum topic: http://forum.terasology.net/threads/module-categories.1068.  Try it out, tell me if it sucks or if it has some potential.

**Overall goal**
Make it easier to select a compatible set of modules

**User experience**
- restarting the game should bring back the create game screen just like you left it
- selecting additional modules should not be reset when changing gameplay
- from a fresh config, default options should be selected
- changing gameplay modes should try not to leave you with an empty world generator
- switching between gameplay modes should attempt to switch to the default world generator for that module