Re #297: Move Categories over to key/keyref from ID/IDREF * Make category use a string for an ID * Add key to system - key needs to be outside <complexType> so we can't put it in warfoundry-cats.xsd at the moment TODO: Add matching code to Race, or find a way to "alias" and create a categories element that uses the type and the key within warfoundry-cats.xsd