Re #324: Add saving of Race and System data to files * Convert IWarFoundryFileSaver from a marker interface into an interface with a purpose of its own so that we can support multiple data files in one zip * Repurpose WarFoundryXmlSaver to be just an Army saver - we still need to make it usable as part of a larger whole * Add a WarFoundryLoadedObject class for objects loaded from files and make Race/Army/GameSystem inherit from it