Replace qVariantFromValue with QVariant::fromValue

Summary: qVariantFromValue is obsolete (https://doc.qt.io/qt-5/qvariant-obsolete.html#qVariantFromValue-1) and causes build failure in KActivities and KEmoticons with current Qt dev

Test Plan: KActivities and KEmoticons build again with current Qt dev

Reviewers: Frameworks, dfaure, apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D21107