Add a utility accessor to get a QUrl from a ResultSet::Result

Summary:
Improve API UI to prevent user handling QString to QUrl.
Would prevent error such as D22005 and D24728 and allow to simplify their code

Reviewers: ivan

Reviewed By: ivan

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24741