Move keep-above clients to the Normal layer when showing desktop

Summary:
Implementation of the Show Desktop feature moves desktop windows to
the Above layer, but it doesn't take into account existing clients
that belong to the Above layer. If there are any, we have to move
them to a layer below (e.g. normal), otherwise those clients will be
visible when showing the desktop.

BUG: 406101

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D20153