PDF multipage view: add vertical scrollbar

So far it does not update the spinbox with the current page to which
the user has scrolled, because ListView.currentIndex doesn't change.

Change-Id: I5dfa644401f77628c71ad1db7d64c5f0ac1e0c65
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/pdf/quick/qml/PdfMultiPageView.qml

@@ -241,6 +241,7 @@ Item {
                 }
             }
         }
+        ScrollBar.vertical: ScrollBar { }
     }
     PdfNavigationStack {
         id: navigationStack