Fixed major loss in performance from tile entities

- Reverted SecretStone to metadata-based textures to improve
performance: thousands of tile entities was causing huge lag
- Added new BlockDungeonStone for creative tab users; this can take on
nearly any texture
- Dungeon Cores still use new tile entity texture system, extending
BlockDungeonStone instead of BlockSecretStone
- Added tooltips for Secret and Dungeon Stone ItemBlocks