[effects/slide] Use FormLayout in KCM

Summary:
HBoxLayout was a bad choice for label-spinbox pairs. Use appropriate
layout(FormLayout) for such things.

Test Plan: The KCM still looks the same.

Reviewers: KWin, mart

Reviewed By: KWin, mart

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D13422