[kcmkwin/compositing] Fix effect video previews

Summary:
Currently, video previews do not work because we try to call showHide()
function that was removed in 787c39cd.

Reviewers: #kwin, #plasma, davidedmundson

Reviewed By: #kwin, #plasma, davidedmundson

Subscribers: kwin

Tags: #kwin

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