Improve plugin caching

Summary:
Extend the plasmoid startup cache to DataEngines and ContaimentActions.
Include a cache for script engines. In practice it's always the same, we don't
need to check the file system every time.

Reviewers: Plasma, davidedmundson, mart

Reviewed By: Plasma, davidedmundson, mart

Subscribers: davidedmundson, mart, broulik, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D22258