Mostly aesthetic changes to the code for the InventoryGUI class. Renamed members more concisely, added comments, made some methods private, removed un-needed methods.
Added class fields for the background colour and text colour of the InventoryGUI.
Fixed a bug where inventory text was not displayed.
Refactored the use of Fonts. A font is now passed to the constructor for the InventoryGUI.
Wrote (templated!) Inventory class to hold the player's inventory. It is not used by the game code yet.