Port away from deprecated qVariantFromValue
Summary:
qVariantFromValue is deprecated since Qt 5.14. It's advised to use
QVariant::fromValue instead.
Test Plan: Compiles.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D24042