keyboard navigation in systemsettings

Summary:
correct keyboard navigation in systemsettings, since there is a mix
of qwidgets and QQuickWidgets, when a QQW takes focus, then the
focus chain in the keyboard navigation should traverse it, then go
put of it and focus the next widget, being a normal qwidget or another
qqw in which case the same hack applies

Test Plan:
navigating in and out the sidebar and the intro page works

QML based kcms are still not managed, they will need a similar thing

Reviewers: Plasma

Subscribers: davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8221