[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