Fix handling of the case where specified surface can't imported
An xdg_imported resource must be created no matter whether there is an exported surface with the specified handle.
This change fixes handling of that case in kwaylandserver by introducing a dummy imported object.
If there is no exported surface with the given handle, the compositor should create an xdg_imported resource and immediately send the destroyed event.
A dummy imported object is needed because generated code sets resource implementation; otherwise we would set an implementation without any user data to mark the resource as inert.
Closes #16 (closed)