[server] Add a method SurfaceInterface::surfaceAt(const QPointF&) ->…

Authored by graesslin on Apr 5 2016, 12:26 PM.

Description

[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