[kcmutils] Port away from deprecated methods in Qt 5.14

Test Plan:
Builds. If there were any unittests, I would have run them...
If there was a maintainer, I would have added him as reviewer...
Many things one could do, with "ifs" ;-)

Reviewers: apol, ngraham, davidedmundson

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T11553

Differential Revision: https://phabricator.kde.org/D23990