Bump to Qt5.12 - Drop kwin's custom ServerSideDecorationManager interface

Test Plan: Still have decorations thanks to xdg-decoration

Reviewers: #plasma, #kwin, zzag, graesslin

Reviewed By: #plasma, #kwin, zzag, graesslin

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18515