Revision of the "books" mod.
* Made bookcases stackable
* when destroyed, bookcases drop their content - be careful with 99-er stacks ;)
* consistent naming (bookshelf -> bookcase)
* package structuring
Introduced MapGenerators that consists of several ChunkGenerators.
MapGenerators/ChunkGenerators can now be placed in mods.
CreateNewWorldUI modified to dynamically load MapGens.
Introduced GameType and GameTypeManager.
Refactored some code.
Example Light and Shadow game type.
Merge PR #626 from @skaldarnar - books revision
Merge PR #621 from @synopia - mapgen/gametype structure overhaul