NSObject Extensions

/// Registers the observer object to receive KVO notifications for the key path relative to
/// the object receiving this message.
///
/// - Parameters:
///   - observer: The object to register for KVO notifications.
///               The observer must implement the key-value observing method
///               `observeValueForKeyPath:ofObject:change:context:`.
///   - keyPath: The key path - relative to the object receiving this message -
///              of the property to be observed.
func addObserver(_ observer : NSObject, forKeyPath keyPath : String)