KWin: tooltips

Summary:
BUG: 383040

KDecoration part of the patch:
https://phabricator.kde.org/D7246

Reviewers: graesslin

Reviewed By: graesslin

Subscribers: cfeck, graesslin, luebking, broulik, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D9294