Laying some basic structure out for adding in CSV-read items
XML is almost certainly overkill for this.
Big, muddled commit:

* Removing ItemFactory, moving its functionality to data + GameStateManager
* Item has a more intuitve/flexibile constructor to be data-driven
* Others changed to use this new system
Moving items to GameStateManager. This commit has basis for rarity.

Need to get back to spawnable items by updating the GameWorldStage to use this.