new logout effect

Summary:
new logout effect: it's just a slow fade in of the logout
greeter (window is usable immediately tough

Test Plan: logout greeter now fades in without setting the window opacity manually

Reviewers: graesslin, #plasma

Reviewed By: graesslin, #plasma

Subscribers: broulik, plasma-devel, kwin, #kwin

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5045