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