[kcm/decoration] Port to KPluginLoader

Summary: KPluginTrader's future is uncertain. KPluginLoader is enough here

Test Plan: Open deco KCM. Themes are listed

Reviewers: #kwin, dfaure

Reviewed By: dfaure

Subscribers: dfaure, zzag, kwin

Tags: #kwin

Maniphest Tasks: T12173

Differential Revision: https://phabricator.kde.org/D25534