Fix warning on ScrollablePage

Summary:
At the moment we'll be going through all of the children objects and reassigning
the overlay as the parent. This is problematic because some objects won't allow
it, getting things like
ScrollablePage.qml:189: Error: Cannot assign QObject* to Action_QMLTYPE_11*

This patch changes it so only Items are reparented

Test Plan: Don't get the warnings anymore

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D26632