Minor Changes

Cleaned up our code related to initializing dimensions. Removed
redundant instances of that code and instead created a function:
PocketManager.loadDimension() - to centralize all uses of that logic.
Added LinkTypes.REVERSE to represent links leading back out of pockets
through their entrances. That distinction might prove critical in the
future when we support resetting dungeons.
Added Support for Missing Link Types

Added functions for generating destinations for some link types that we
hadn't included yet - player-made exits, trapdoors, and dungeon exits.
The player-made and dungeon exits still don't generate anything, but
there's a single function where we can write up the code:
DDTeleporter.generateSafeExit(). Also added some simple code for listing
root dimensions in PocketManager. None of this has been tested yet!