[editor] Generate event for tab change (selected page index)

projects/editor/src/app/components/unit-view/unit-view.component.html

@@ -12,7 +12,8 @@
 
     <mat-drawer-content>
       <mat-tab-group mat-align-tabs="start" class="mat-tab-fill-height"
-                     [selectedIndex]="unitService.selectedPageIndex | async">
+                     [selectedIndex]="unitService.selectedPageIndex | async"
+                     (selectedIndexChange)="unitService.updatePageSelection($event)">
         <mat-tab *ngFor="let page of (unitService.unit | async)!.pages; let i = index" label="Seite {{i+1}}">
           <ng-template mat-tab-label>
             <mat-icon *ngIf="page.alwaysVisible" class="page-alwaysVisible-icon">assignment</mat-icon>