Serialize item slots

[brrritssocold]

Uses serialization for the Players inventory (carried on person).

• Make ItemSlot serializable
• Convert itemslots array to vector
• Move serialization for item slots to player
• Remove legacy item slot variables (itemslot1, itemslot2, ...) from code (replaced with get functions for compatibility)
• Use vector serialization to remove duplicated item slot code
• Removed item slot switching from UrtaQuest as it was pointless - item slots are variables of the player instance, which already gets switched