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

Summary:
Tweak spacing and margins, improve the look of headers and descriptions.
{F7155024}

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