Added variables for deciding when to add new aliens in the game scene
Added variables for deciding when to add new aliens in the game scene

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