[effects/fadingpopups] Don't animate KDE Plasma splash screen

Summary:
In order to be compatible with the monolithic Fade effect, this effect
animates special windows (e.g. notifications, etc) as well, but some
of those special windows shouldn't be animated by this effect (e.g.
KDE Plasma splash screen).

Test Plan:
Started new Plasma on Wayland session, the KDE Plasma splash screen
smoothly faded out without flickering/blinking, etc.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17209