Don't have a non-integral size on separators

Summary:
They look odd when placed by a rectangle in a Layout, they will be showing an
empty space when scaling between the element and the one after.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D12056