Updated State system (Bevy 0.12)

[flatypus]

Adding on to #238

Defining enum for the game, with #[default] defining the initial state

#[derive(Debug, Default, Clone, Copy, Eq, PartialEq, Hash, States)]
enum GameStates {
	#[default]
	GameStart,
	Loading,
	InGame,
	...
}

When setting the state on init, use:

App::new()
	.add_state::<GameStates>()

and the state will be set automatically to default (GameStart)

Rest of the changes in the issue linked are up to date as far as I know.