libdoomsday|DEDRegister: DEDRegister provides immutable access to lookup DictionaryValues
libcommon: Traverse the DEDRegisters of Episode and MapInfo defs

As of this commit, the build completes without error. It is however
most certainly broken. The next step is implement parsing of Episode
defs and move that data out of IdTech1Converter to new DED files for
respective game plugins.