A lightness KVO helper class.
Avoid excessive KVO code.
Single keypath observer
- (instancetype)initWithObject:(id)object keyPath:(NSString *)keyPath target:(id)target selector:(SEL)selector;
Keypaths perform one selector
- (instancetype)initWithObject:(id)object keyPathList:(NSArray *)keyPaths target:(id)target selector:(SEL)selector;
Keypaths perform different selectors
- (instancetype)initWithObject:(id)object keyPathSelectorMap:(NSDictionary *)map target:(id)target;