Combines AssetsManager, ConfigManager, and StateManager for a manifest into one convenience struct.
AssetsManager< Manifest > AssetMgr
AssetsManager typed to this manifest — access textures, fonts, sounds.
StateManager< Manifest > StateMgr
StateManager typed to this manifest — access the manifest's State enum.
ConfigManager< Manifest > ConfigMgr
ConfigManager typed to this manifest — access string/int/float/bool config.