Implement LevelGroup

This should resolve #91.

The LevelGroup class works similarly to the UserProfile class in that it involves serializing and deserializing data from an XML file. This particular file, which is levellist.xml, contains an ordered list of paths and names for each level.

...uh hopefully it works?

(Note: Bug 46 isn't in the levellist.xml since I manually took it out, although it's easy to add back in if one really wants to...)