[kcmkwin/compositing] Remove effect list item selection, fix list item size...

[kcmkwin/compositing] Remove effect list item selection, fix list item size after hiding the effect video, use a real button as the play button and use the right busy indicator

Summary:
Makes some improvements to the Effects KCM (details in title).
{F6464106}
{F6464355}

Test Plan: Open the Effects KCM.

Reviewers: #kwin, #vdg, ngraham, davidedmundson

Reviewed By: #kwin, #vdg, ngraham, davidedmundson

Subscribers: davidedmundson, ngraham, #vdg, kwin, #kwin

Tags: #kwin, #vdg

Differential Revision: https://phabricator.kde.org/D17416