Item system

[radeusgd]

• Load ItemTypes: JS files with properties and functions.
• Item class (ItemType + unique metadata)
• save items laying in world in chunk-metadata table
• container utilities in SE (width, height, item array)
• save items in special items or player inventory array

[radeusgd]

Above commit does most of it. Only one left is containers, but that will be made later, because it requires UI to work OK, and interactions also.