[FormLayout] Use even top and bottom spacing for separator

Summary:
The Kirigami separator has spacing below it, but not above it when in a Form Layout. This patch adds a check if there is a separator present and if it is, it adds spacing above it.

BUG: 405614

Test Plan:
Before:

After:

Reviewers: mart, Kirigami, ngraham

Reviewed By: mart, Kirigami, ngraham

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D20585