Add PdfLinkModel

Internal links and web links populate the QALM, which can then
be used to drive a Repeater to position highlight rectangles
with TapHandlers, which will handle a click by jumping to the
link destination.

Fixes: QTBUG-77511
Change-Id: I3b5b96d6e82bfd578f31f631f24279173036a080
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>