[libkwineffects] Add signals windowShown and windowHidden to EffectsHandler

Summary:
This allows effects to animate when a window is shown again and when
a window gets hidden but not yet closed/destroyed. This situation
happens on X11 for e.g. auto hiding panels and on Wayland for pretty
much any window which properly unmaps (windowHidden) prior to destroy.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2084