Add QPdfDestination; NavigationStack stores page, location and zoom

Push/back/forward behavior seems more correct now, but still no
autotest yet.

QPdfDestination might be useful to represent locations of search results,
for link destinations and maybe named destinations too.

Fixes: QTBUG-77512
Change-Id: I113b2c535a2cd302106e6546104c64e12985d387
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>