Doomsday Engine

Refactor: PathTree specialization

There is now a way to specialize PathTree with custom nodes. libdeng2
provides some basic utilities:

- PathTreeT<> template
- UserDataNode (node with a user value and pointer)
- UserDataPathTree (typedef of PathTreeT<UserDataNode>)
- PathTreeIterator<> template for iterating path tree nodes (Qt style)

Updated engine to use UserDataPathTree and PathTreeIterator<> where
appropriate.