[libkwineffects] Add keepBelow property to EffectWindow

Summary:
EffectWindow has keepAbove property, but not keepBelow.

This change adds keepBelow property as a counterpart to keepAbove.

Test Plan: Manually.

Reviewers: #kwin, mart

Reviewed By: #kwin, mart

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13650