Summary:
When user asks KWin to show desktop, the compositor just raises every
desktop window, it doesn't attempt to minimize normal clients, etc.
Because desktop windows contain stuff like background and icons, the
compositor has to raise all clients that are somehow correlated with
desktop (e.g. panels, etc). Otherwise it would pretty weird to show only
desktop background.

Currently, not all plasmashell's clients are raised when the compositor
shows desktop because belongsToDesktop is not implemented for ShellClient.

BUG: 404801

Test Plan:
* Show desktop;
* Open Kickoff (without this patch Kickoff is not visible).

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D19386