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:

https://www.youtube.com/watch?v=XiLq7EAVCp0

After:

https://www.youtube.com/watch?v=cH_Ki-sqY8M

=== Scale effect

Before:

https://www.youtube.com/watch?v=Eb2a3U7R10I

After:

https://www.youtube.com/watch?v=4AKu3fdrnYQ

=== Sheet effect

Before:

https://www.youtube.com/watch?v=xPPSnR5FUU0

After:

https://www.youtube.com/watch?v=o_hxTNT-5Hg

=== Popup menus on Wayland

Before:

https://www.youtube.com/watch?v=5DnrY8p3F5A

After:

https://www.youtube.com/watch?v=7XEo8n_CrCc

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davided...