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