Add KPropertySkeletonItem

Summary:
This new item allows to use a QObject property as a source for a
setting. This is especially convenient for using with KCMs present in
systemsettings which tend to store information outside of KConfig (for
interfacing deeper with the system).

Reviewers: Frameworks, dfaure, davidedmundson, bport, crossi

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25211