Conditionally show navigation buttons in the header for multi-page KCMs

Summary:
This makes the navigeability of multi-page KCMs completely obvious. The buttons only show
up in multi-page KCMs where you've navigated into a sub-page, so there is no visual
regression for anyone else.

Test Plan:

Reviewers: VDG, Plasma, mart

Subscribers: GB_2, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25049