Prefab Overhaul

Prefab components are back. PrefabManager loads them (now based off MVC)
according to their Prefab component. This lets us filter, refine, sort,
and organize prefabs.

Currently we're able to sort prefabs by their type (ground, corner,
thing).

TODO:

categories such as "wall", or "tree"
sets that let us organize common sprites (like a set of wall sprites)
and primary/alt tags (primary tile, and alternative looks)