New Classes:
WorldHistory
  writeToWorldWithUndo
  performUndo
  cullUndoLayers
- test using stubs for EntityPlayerMP and WorldServer.
  tested in WorldSelectionUndoTest

WorldFragment
  readFromWorld
  writeToWorld
- all tested by InGameTester

WorldSelectionUndo
  makePermanent
  writeToWorld
  readFromWorld
- all tested by InGameTester
- (one test in WorldSelectionUndoTest)

