sw3: GameItem Standardization and Serialization

[hqkirkland]

ClothingItem and AvatarItem aren't great separators: they're basically the same thing, except AvatarItem is a typedef with extra game client-specific data (file path, item color). Here, AvatarItem serves as the best model for what an "owned item" should look like vs. a general, non-owned GameItem concept.

GameItem class should be built from the ClothingItem class.
ClientData.clothingItems should be set by filtering GameItems into it by clothing type.
Inventory.wardrobe should be a set of all GameItems that the user OWNS, each translated into an AvatarItem.