The main motivation for this change is to enable support for our
proprietary shadow protocol in LayerShellV1Client.
Previously we couldn't move code that handles shadows in WaylandClient
because WaylandClient::bufferGeometry() was a pure virtual method.
The layer-shell protocol allows wayland clients to create surfaces that
can be used for building desktop environment components such as panels,
notifications, etc.
The support for the plasma-shell protocol will be dropped once plasma in
all its entirety is ported to the layer-shell protocol.