UUID support

Changes made:

- Cut out legacy file conversion routine (server admins will have to upgrade in two steps from very old versions)
- Store chest inventories by UUID internally
- Save chest in filename by UUID
- Convert existing chests on the fly
- Give warning about chests with unknown UUIDs

Caveats:

- Chests with arbitrary names cannot be opened any more (gives not-found error)
Too much whitespace ;-)