Fixed some issues related to pausing and game over.
- All pause elements are now removed from the screen when the game over screen is displayed.
- GameController's pauseGame method now calls super.pause to set the isPaused flag to true. This means we handle a user pause the same as if our app is minimized, which currently seems to be fine.
- Made sure we don't show any pause elements during game over, even if app is minimized, but that we still are keeping track of the app's pause status.