adding the Key item

new file:   assets/textures/key.png
modified:   src/item/Battery.java
modified:   src/item/ItemListEnum.java
new file:   src/item/Key.java
modified:   src/rendering/TextureManager.java
updating inventory to handle and keys and batteries

modified:   src/entity/player/Player.java
modified:   src/userInterface/inventory/Inventory.java
modified:   src/userInterface/inventory/InventoryItemEnum.java