Summary:
It's only needed by the standalone x11 variant. This allows us to
simplify the creation of the OpenGLBackend: it's created by the
platform plugin - we don't need custom complex logic.
Reviewers: #plasma
Subscribers: plasma-devel
Projects: #plasma
Differential Revision: https://phabricator.kde.org/D1392
All the functionality of Overlay Window is moved to its own class
OverlayWindow. It is created and owned by class Scene, since almost
all function calls are called from this class.
REVIEW: 101866