[ACA-3899] Viewer thumbnails cannot be accessed by keyboard (#6150)

* accessibility

* add thumb focus

* close thumbs list event

* fix eventbus listener

* rename parameter

* track previous focused element

* implement FocusKeyManager

* fix getGlobalEventBus is deprecated

* keyboard navigation

* set tabindex

* setActiveItem on list change

* update  test

lib/core/viewer/components/pdf-viewer-thumb.component.spec.ts

@@ -66,4 +66,12 @@ describe('PdfThumbComponent', () => {
             done();
         });
     });
+
+    it('should focus element', () => {
+        component.page = page;
+        fixture.detectChanges();
+        component.focus();
+
+        expect(fixture.debugElement.nativeElement.id).toBe(document.activeElement.id);
+    });
 });