[kded] Separate configuration storage path

Summary:
As a preparatory step save the configuration storage directory in a separate
subfolder variable. In KDE6 this should go into a separate subfolder 'config'
of the current data directory.

Test Plan: Autotest updated.

Reviewers: Plasma

Subscribers: davidedmundson, broulik, plasma-devel

Tags: Plasma

Maniphest Tasks: T10028

Differential Revision: https://phabricator.kde.org/D16990