[spec] Typings on notifications

[Speedy37]

The notification center is actually not typed, so there is no compile time verification about typings with events.

To type the notification center I propose to add typings informations to the event: the event would not be a "string" anymore pour an object holding type informations.

export interface Notification<ObjectType extends Object, UserInfoType extends {} | undefined> {
  name: Event<ObjectType, UserInfoType>;
  object: ObjectType;
  info: UserInfoType
}

export class Event<ObjectType, UserInfoType> {
  constructor(public name: string) { }
  Notification: Notification<ObjectType, UserInfoType>;
  toString() { return `<Event:${this.name}>`; }
}

let notificationCenter = new NotificationCenter();
let myEvent = new Event<MyEmitClass, { a1: string }>("myEvent");

class MyObsClass {
    register() {
        notificationCenter.addObserver(this, "onMyEvent", myEvent);
    }
    onMyEvent(notification: typeof myEvent.Notification) { }
}