KPageWidgetItems live in a multi-page KPageDialog that already shows the current page's
title in its page selector sidebar/tab bar/whatever. Showing it again on the page is
redundant. There's no toolbar where the title could live like in Kirigami, either.
RFC because this is mostly to solicit comments. In principle I think this change makes
sense, but when trying it out, not having a header on the page does feel a little bit
strange to me.
If we do decide to do this, obviously this is only one of several required patches
(for Plasma, System Settings, etc)