support for multi pages kcms

Summary:
support multiple levels for QML kcms.
use a Kirigami.ApplicationItem, so qml kcms can now use the full Kirigami api
and use the application's pagerow for their levels.
The breadcrumb of levels is supposed to be shown by the systemsettings shell.
Depends on D14579

Test Plan:
Old kcms still work as they did
a mqml kcm can push multiple pages

Reviewers: Plasma, Frameworks, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: abetts, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14580