GUI: Inventory/Containers

This MR adds a basic inventory and so called generic containers (e.g. chests).

Changes:

• improved item rendering #66 (closed)
• fix a ton of errors
• refactor container api
• refactor ItemStack api
• more changes and fixes

ToDo:

• container actions
  • swap
  • pick all (only click checking left)
  • distribute
  • reverting of actions
• GUIManager: pop specific element
• Implement all containers + gui