PdfSearchModel: be QALM and find search results on all pages

It's a QAbstractListModel, so now PdfMultiPageView has a ListView in a
left-side Drawer showing all results found so far.

- In PdfMultiPageView, multiple pages exist at once, so it makes sense
  to use the same searchmodel for all.
- It's faster and saves memory.
- Search results on each page can be cached.
- It's possible to show search results in a ListView or QListView.

Change-Id: I66fba6975954a09a4d23262be87ff8cc25ee7478
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>