Design Inventory and Item Pickup system (initial design)

[aPruner]

• Decide on design (class hierarchy, implementation) for items in inventory vs item pickups in the game world (GameObject items)

[aPruner]

See wiki page (TODO) for class hierarchy. This PR will also include relevant derivations for item subtypes such as weapons, armor, potions, etc

[aPruner]

WIP PR opened: #24

[aPruner]

Out of this issue, I will have to spawn many more. The scope of this issue is way too large as there is a lot of coupling that will have to be sorted out.

A few examples:

• The ItemPickup system will be coupled to the room/dungeon system in some way (as items have to have a location in the dungeon/room), which needs to be fixed first.
• The Inventory system will be coupled to an InventoryGui system which also needs to be designed and built (thankfully the Gui framework is there now). This Gui system will also need to be able to handle mouse events for dragging and dropping items around, as well as item tooltips, etc
• The Inventory system, as it is built right now, can only handle adding and removing items with a stackQuantity and maxStackQuantity of 1. That is, items that cannot be stacked on eachother.
• The ItemPickup behaviour needs to be implemented. That is, when the player walks over an item (or clicks on it, or something) the player picks it up and it appears in the inventory.