[effects/fadedesktop] Rewrite it

Summary:
The primary reason for rewriting this effect was to clean up code and
fix spawning of multiple animations for a single window when user cycles
through virtual desktops very quickly.

Visually, the rewritten version doesn't deviate from the old version.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16454