NoteObserverManager

interface NoteObserverManager

Observer manager

notifyObserver	Notifies all observers about update `abstract fun notifyObserver(): Unit`
registerObserver	Registers observer `abstract fun registerObserver(observer: NoteObserver): Unit`
removeObserver	Removes observer `abstract fun removeObserver(observer: NoteObserver): Unit`

Manages Notes and NoteObservers

class NoteManager : NoteObserverManager, NoteActionObserver