Add some std::move in setter functions

Summary:
typically we pass the param as const &, but here we forgot, we can fix
that problem by adding an std::move and have a very similar effect to if
we had declared the params as const &

Reviewers: leinir

Reviewed By: leinir

Subscribers: leinir, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24395