SafeAreaExtension 모든 UIView의 safeAreaInsets변화 감지 Extension 설명: https://swifter.kr/2017/10/17/safeareainsets-%EB%B3%80%ED%99%94%EC%97%90-%EB%8C%80%ED%95%9C-%EA%B0%90%EC%A7%80-%EB%B0%A9%EB%B2%95%EB%93%A4/ MIT License.