UI fixes for logout dialog

Summary:
* use backgroundcontrast effect, improves readability a lot
* non-current actions a bit more translucent
* background is actually black if the color scheme is dark, looks washed out otherwise
* add an OK button, that was complained a lot
* short fade animation at the beginning

Test Plan: see screenshot

Reviewers: #plasma, sebas, broulik

Reviewed By: #plasma, broulik

Subscribers: graesslin, broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5036