[player] Add NavigationService for page navigation

belongs to commit #f6668bdd

projects/player/src/app/services/navigation.service.ts

+import { Injectable } from '@angular/core';
+import { Observable, Subject } from 'rxjs';
+
+@Injectable({
+  providedIn: 'root'
+})
+export class NavigationService {
+  private _pageIndex = new Subject<number>();
+
+  setPage(pageIndex: number): void {
+    this._pageIndex.next(pageIndex);
+  }
+
+  get pageIndex(): Observable<number> {
+    return this._pageIndex.asObservable();
+  }
+}