Keep current scroll position when reloading page

Summary:
I don't have an environment supporting QWebEngine so I need some help
here. Patches and testers welcome.

BUG: 391981
FIXED-IN: 5.0.2

Test Plan: Compile and run with QWebKit and QWebEngine

Reviewers: wojnilowicz, asturmlechner

Reviewed By: wojnilowicz

Differential Revision: https://phabricator.kde.org/D11450