Kirigami Pages are required for kcms

Summary:
adapt to the new multilevel stuff as it's needed for qml kcms to
inherit in some shape Kirigami.Page in order to have a title
this was the only kcm left which wasn't a page.
it needs this change with new frameworks and workspace combination
but old workspace and new frameworks is on as old systemsettings did the title itself

Test Plan: looks the same and still works

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: abetts, ngraham, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15121