Implement better loot/interraction system

[Abb4]

followup for #3 where Collectible was added

When interacting with loot, add gold to players gold.

Maybe instead of a gold number also create a basic inventory list. The valuables in this list are added together at level end, spelunky style. It is a little more satisfying.

Maybe also build a foundation for interacting with other various objects (e.g. doors) here too.

• [x] maybe try to avoid inheritance of classes (interface inheritance ok), try something like Collectible extends IInteractible
• [x] maybe use the visitor pattern extensively
  • player could implement IInteractionEntity.GetInteractionEntity which is passed into IInteractible.GotInteractedBy(IInteractionEntity)
  • this would completely decouple IInteractionEntities (such as player, enemy, even spells) from IInteractibles, such as doors and objects
  • I am unsure if this amount of decoupling is even needed or would even be harmful, testing needed