Keep Deleted transients above old parents

Summary:
If a modal window is closed, usually, it will go behind its parent. The
reason for this is that Workspace::constrainedStackingOrder() puts only
AbstractClient transients above parents, not Deleted transients.

So, if fade/glide/scale effect animates the disappearing of a transient,
unfortunately, one can't see that animation.

BUG: 397448
FIXED-IN: 5.15.0

Test Plan:

Closing of a transient and parent window

Before:

After:

Scale effect

Before:

After:

Sheet effect

Before:

After:

Popup menus on Wayland

Before:

After:

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: abetts, davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D14868