[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