introduce concept of header and footer for kpageview

Summary:
put the dialog buttons in the footer for kpagedialog
this is the first step for the new setting dialog windows.
note that the look is still not that, but the changes will have to
come probably from Breeze

Test Plan: tested some application f=config windows

Reviewers: Plasma, Frameworks, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: hpereiradacosta, cfeck, ndavis, ngraham, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T11153, T11279

Differential Revision: https://phabricator.kde.org/D22083