Required information is passed through the ctor of Private.
::create is still virtual as ShellSurfaceInterface is retrieving
client information. This can be fixed once we have a better wl_client
encapsulation class.
The Resource base class is supposed to be used by all interface
classes which get created for a wl_resource.
Most interface classes are adjusted, but there are some exceptions:
* BufferInterface: is different as the wl_resource is already created
* PointerInterface and KeyboardInterface: those two need changes, the
implementation differs from all other interface implementations.
