[effects/slidingpopups] Use single QHash to store animations

Summary:
This patch intends to simplify "management" of animations and reduce
QHash abuse.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14096