[kcmkwin/compositing] Fix effect video previews

Summary:
Currently, video previews do not work because we try to call showHide()
function that was removed in 787c39cd420ac1af856765ed18f01271ece045db.

Reviewers: KWin, Plasma, davidedmundson

Reviewed By: KWin, Plasma, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18720