Emit windowShown and windowHidden from Client::internalKeep and ::internalHide

Summary:
With this change auto-hiding panels are animated again by SlidingPopups
effect.

Test Plan: Test case adjusted, and tested in VM

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D2157

BUG: 354407
FIXED-IN: 5.8.0