Refactored the Inventory GUI code into a new class. The class is not used in the game code yet.
Fixed ridiculous, unreasonable typo in function name.
Refactored the way the inventory background is drawn, to fix an issue with class inheritance. It is now a RectangleShape remade every frame, rather than a persistent Sprite.
Altered GameWindow to create and use an InventoryGUI object, and removed previous code in GameWindow for drawing the GUI.
Added private fields to the Inventory GUI for its drawing parameters, and moved the defaults from Constants.h to inside the class.