`WindowPlugin` exit systems moved to `Last`

bevy::window::close_when_requested, bevy::window::exit_on_all_closed and bevy::window::exit_on_primary_closed have all been moved into the Last schedule to prevent systems that run after Update and rely on windows existing from panicking on the last frame of the application.

exit_on_all_closed and exit_on_primary_closed have also been added to a new SystemSet, ExitSystems.