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