[effects] Adjust SlidingPopups to new windowShown and windowHidden signals

Summary:
With this change SlidingPopups is able to animate Wayland clients
properly, though windowHidden does not yet work for OpenGL based
windows (buffer seems to get lost somewhere).

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2085