[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