Add property 'Item.DefaultSkinId'

[sliekens]

Right now, only equipment item types have a DefaultSkin property:

• Armor.DefaultSkinId
• Weapon.DefaultSkinId
• Backpack.DefaultSkinId
• GatheringTool.DefaultSkinId

This is not in line with the json schema, which defines field default_skin on the root object.

I now believe that the game is not limited to what item types can have a default skin, and that any item type can be used to unlock a skin. It's easy to imagine a consumable item type that unlocks a skin but cannot itself be equipped.