Adjust all components to use the QSharedPointer. Aurorae obviously has
to create a QSharedPointer, Shadow holds a QSharedPointer for the
DecorationShadow (advantage: is kept when the Decoration is destroyed),
and the KCM needs to add a property on PreviewItem to get access to the
Shadow. It's no longer a Q_PROPERTY on Decoration and we cannot re-add
it as a dynamic property (cannot be read from QML side).
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