[server] Add a method SurfaceInterface::surfaceAt(const QPointF&) -> SurfaceInterface*
Summary:
The new method returns the (child) surface at a given surface position
taking care of stacking order, whether surfaces are mapped, etc.
Reviewers: Plasma
Subscribers: plasma-devel
Projects: Plasma
Differential Revision: https://phabricator.kde.org/D1319