Refactored inventory and equipment

To take advantage of new asset type.
The format of asset is not yet fully determined, but for now I changed
it to ordinary TArray from TMap, and access elementes directly by their
index.

Array is 100% static, you never should add or remove items at runtime
from it.
You also never should reference items directly by their index. There
will be special item picker, for creating loot actor, which will allow
you do pick items from asset and assign them to actor, which in turn
will make possible to loot them, and add to player inventory.