Added an item for inspecting entities powered by an external reference.

The item can currently only be obtained via /giveItem inspectionTool

The item has two purposes:
- You can link it to an entity and then use that item to view the current
  json representation of the selected entity. To get started use the item
  and enter "this". This will make the inspection tool show it's own
  entity data in json format. Via the owener attribute you can then
  determine the ids of the other items in your inventory.
- It is powered by a component that has an reference to the selected
  entity. That makes it possible to construct external entity references
  accross chunk borders. Thus the item can be used to debug/test external
  entity references too.
Merge PR #1406 by @flo - debug item