make push/pop part of ConfigModule API

Summary:
for better separation, abstract push/pop and columnWidth in the ConfigModule class so
the qml part won't have access to ApplicationItem (which will be eventually
sandboxed anyways)

Test Plan: tested a basic mock kcm with multiple columns

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: davidedmundson, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D15098