Basic Configurable Dungeon Chains

Completed a basic version of configurable dungeon chains. Almost all of
the final funcionality is present. However, the configuration is
hardcoded at the moment, not read from a file. This was done for testing
purposes. I'll add reading from config files soon.

Dungeon packs are partially implemented. Built-in and custom dungeons
are currently thrown into the default pack, Ruins. The next step is to
generalize the dungeon registration code in DungeonHelper so that we can
detect dungeon packs, read their config files, and register dungeons
with their corresponding pack. dd-export will need to support packs as
well. dd-rift will have issues dealing with duplicate dungeon names
across packs, but this isn't a major concern and can be dealt with in
the long term.