Port containmentActions to use KPlugin instead of KService

Summary:
It saves us from going through the deprecated (slower) path and will leverage the PluginLoader lookup cache.

Depends on D22267.

Test Plan: Started plasma, containment actions are still found

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22268