[kcmkwin/compositing] Move show desktop effects to their own category

Summary:
We have two exclusive categories in the Appearance category:
minimize-animations and show-desktop. But it's hard to draw a line
between them, i.e. one can't say whether given effect belongs to the
minimize-animations category or to the show-desktop category.

This change moves show desktop effects to their own category so we have only
one exclusive category in the Appearance category.

Before:
{F6160592, layout=center, size=full}

After:
{F6161173, layout=center, size=full}

Reviewers: #kwin, #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: davidedmundson, graesslin, ngraham, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14458