Add the missing api for multilevel KCMs to control the columns

Summary:

• add currentIndex and depth, which would map to the view in SystemSettings using a Kirigami PageRow (but not necessarly)

• On multilevel kcms, Fix layout alignment issues that would prevent

ScrollViewKCM and SimpleKCM when displayed one besides the other to look
aligned with each other

Test Plan:
tested most current QML kcms, tested the new features extensively
with the QML networkmanager kcm rewrite

Reviewers: Plasma, jgrulich

Reviewed By: jgrulich

Subscribers: abetts, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17350