Take the scrollbar into account for the ScrollablePage margin
Summary: BUG: 401972
Test Plan: Now I never get that overlapping, it could have other ramifications though.
Reviewers: Kirigami, mart, ngraham
Reviewed By: Kirigami, mart, ngraham
Subscribers: ngraham, plasma-devel
Tags: Kirigami
Differential Revision: https://phabricator.kde.org/D17551