[effects/slide] Clean up

Summary:
* Reorder and sort includes
* Use EffectWindowList
* Fix namespace comments

(this change doesn't add any useful features or bug fixes)

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13979