GameManager Class for SceneObject Management

Create a new GameManager class that takes the create[...]() function calls and all other SceneObject management functionality from GameInstance. In addition to SceneObject management, this class should be able to save/load scenes defined by .yaml files. This should make it easier to create and save scenes in the upcoming editor functionality.

GameManager should be a global class instance like GameInstance, but we should also create a GameScene class that actually holds the scene data.