make use of the new currentIndex api of QML kcms

Summary:
sync kcm currentindex with the pagerow's
depends from D17350

Test Plan: tested with newtowrkmanager qml rewrite of the KCM

Reviewers: Plasma, jgrulich

Reviewed By: jgrulich

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17351