task: deprecates EventEmitter and replaces it with Subject

[JsantosDK]

Closes #1513

[JsantosDK]

@CodeAndWeb PR should be done, and no issue that I can see with the replacement.
Could you please have a look and merge if all is ok?

Something that I also noted, and wanted to be more aware.
Right now, the subjects/event emitters are directly exposed by the service.
This means that the consuming app can do:

this.translateService.onDefaultLangChange.emit({ lang: 'foo', translations: ''})

For me, this seems like an error.
Wouldn't it be better to expose these has observables to the outside, and replace with the following:

private readonly _onDefaultLangChange:  Subject<DefaultLangChangeEvent>;
public readonly onDefaultLangChange: Observable<DefaultLangChangeEvent>;

constructor(...) {
  ...
 this._onDefaultLangChange = this.store.onDefaultLangChange;
 this.onDefaultLangChange = this._onDefaultLangChange.asObservable();
}