[effects/fadedesktop] Set animation as a fullscreen effect

Summary:
This lets other effects know whether they should inhibit or not which
reduces
visual glitches of overlapping animations.

BUG: 321201
FIXED-IN: 5.15.0

Test Plan: Switched desktops, they still faded

Reviewers: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15474