Add convenience for defaults/dirty states to KCoreConfigSkeleton

Summary:
It allows to verify if all the items of the skeleton are in their
default values or if they hold any value deviating from the latest
loaded values from KConfig.

We didn't really need this during the KCModule/QtWidgets time since we
could write KConfigDialogManager just fine without it. But for use with
QML and aiming at having similar magic in KQuickAddons::ConfigModule
such convenience functions will be needed.

Reviewers: Plasma, Frameworks, dfaure, mart

Subscribers: apol, kossebau, davidedmundson, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24494