[KWidgetAddons] port to non-deprecated Qt API
Test Plan:
Added "cxxflags -DQT_DISABLE_DEPRECATED_BEFORE=0x060000" to
kdesrc-build's kf5-frameworks-build-include (in the frameworks module-set)
Reviewers: cfeck
Reviewed By: cfeck
Subscribers: dhaumann, aacid, kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D23797