Stores SaveState model objects with Core Data, and displays the available save states in SaveStatesViewController.
Currently, tapping + button creates a new save state, while tapping a collection view cell loads the save state, regardless of whether Save State or Load State was selected from the pause menu.
# PauseViewController has been renamed to PauseMenuViewController.
# A new PauseViewController maintains a child UINavigationController, which has PauseMenuViewController as the root view controller.
Additionally, refactored GridCollectionViewCell to no longer use a NIB. This makes it easier to reuse for iOS and tvOS, and allows us to use storyboard segues (which include peek & pop)
