Add PdfScrollablePageView, use it in the pdfviewer example

PdfPageView might be useful in some cases, but we need to get feature
parity with PdfMultiPageView as much as possible, including scrollbars.
Including them in the view is convenient, but also less flexible.

Change-Id: Ibbe6a090a5f5b1d340124986fe49672d682ddedb
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>