[kcmkwin/kwineffects] Improve the look of the Desktop Effects KCM list items

Summary:
Tweak spacing and margins, improve the look of headers and descriptions.

Test Plan: Open the Desktop Effects KCM and hover over a list item.

Reviewers: KWin, Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, VDG, Plasma, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D22914