[effects/logout] Animate the disappearing of the logout screen

Summary:
Currently, the fade effect animates both the appearing and the
disappearing of the logout screen. We don't want that because
the logout effect should do that.

D14582 addresses that problem by adding "ksmserver ksmserver" window
class to the blacklist of the fade effect. With that change, only the
logout effect animates the appearing of the logout screen. But there is
a problem... If user clicks the cancel button, the logout screen
instantaneously disappears, instead of smoothly fading out.

This change addresses that problem by adding "out" animation.

Depends on D14582

Test Plan:
{F6175011}
//Now, only the logout effect animates the appearing and the disappearing of the logout screen.//

{F6175013}
//No smooth transitions when the logout effect is disabled.//

Reviewers: #kwin, #plasma, #vdg, davidedmundson

Reviewed By: #kwin, #plasma, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D14592