Allow ManagedConfigModule derived class to register explicitly KCoreConfigSkeleton.

Summary: Helpful for derived class that do not store KConfig settings as children items.

Test Plan: Should not break binary compatibility. Same behaviour as before.

Reviewers: Plasma, Frameworks, ervin, bport, davidedmundson, mart, meven

Reviewed By: ervin, bport

Subscribers: meven, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26046