Added a per-save config file called DimDoorsWorld.cfg. Its corresponding
class is DDWorldProperties. This class supports whitelisting and
blacklisting dimensions for Rift Cluster and Rift Gateway generation.
Note that our ban against generating gateways in the Nether and The End
still applies regardless of those settings. The new config file is
loaded before the server starts initializing terrain. Also moved
DDProperties to another package alongside DDWorldProperties - that
required updating references in most of the mod's files.
