ContainmentAction: Fix loading from KPlugin
Summary: At the moment it was enabled in the PluginLoader but we were not using it anywhere.
Test Plan: See patch in plasma-workspace.
Reviewers: Plasma, Frameworks, mart
Reviewed By: Plasma, mart
Subscribers: kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D22267