v1.16.3_R16

• added pluginKey(key) as a new shortcut for NamespacedKey creation
• PersistentDataHolders and ItemStacks now offer markings:
  • mark(key) marks the object with the given key (conflicts with other plugins are not possible)
  • unmark(key) removes the key from the objects' markings
  • hasMark(key) return true, if the key is among the objects' markings