Added action for the alien: transition to the game over scene if the alien went offscreen
Added action for the alien: transition to the game over scene if the alien went offscreen

nguyenantinhbk77/practice-swift,hanhailong/practice-swift,domenicosolazzo/practice-swift,nguyenantinhbk77/practice-swift,domenicosolazzo/practice-swift,hanhailong/practice-swift,hanhailong/practice-swift,domenicosolazzo/practice-swift,nguyenantinhbk77/practice-swift,domenicosolazzo/practice-swift,nguyenantinhbk77/practice-swift,hanhailong/practice-swift