[effects/logout] Animate ksmserver-logout-greeter

Summary:
On Wayland session, the logout screen has "ksmserver-logout-greeter
ksmserver-logout-greet" window class, so let's animate it too.

Test Plan:
* Started Wayland session;
* Pressed Ctrl+Alt+Del;
* (the logout screen smoothly faded in);
* Clicked the Cancel button;
* (the logout screen smoothly faded out)

* Started X11 session;
* Pressed Ctrl+Alt+Del;
* (the logout screen smoothly faded in);
* Clicked the Cancel button;
* (the logout screen smoothly faded out)

Reviewers: #kwin, #plasma, davidedmundson

Reviewed By: #kwin, #plasma, davidedmundson

Subscribers: davidedmundson, abetts, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14848