World GetDirectSpaceState() return Null [Mono]

[khairul169]

Operating system or device, Godot version, GPU Model and driver (if graphics related):
Godot engine 3.0 master 195b122

Issue description:
Cannot get direct space state for raycasting.. tried to do several method like World.GetDirectSpaceState() and PhysicsServer.SpaceGetDirectState(World.GetSpace()), but both return Null.

This is also happened with Physics2DDirectSpaceState.

Steps to reproduce:
Create a c# script, and paste this code:

World WorldRef = GetViewport().FindWorld();
GD.print(WorldRef.GetDirectSpaceState()); // Null
GD.print(PhysicsServer.SpaceGetDirectState(WorldRef.GetSpace())); // Null

Console output:

ERROR: CSharpLanguage::alloc_instance_binding_data: Parameter ' type_class ' is null.
   At: modules\mono\csharp_script.cpp:659
DSS: Null

Link to minimal example project:

[neikeq]

The problem is that classes like PhysicsDirectSpaceStateSW are not registered manually. Instead, _post_initialize will register them when an instance is created. Bindings generators cannot detect many of these classes because of that.

I think the best solution would be the following:

• Only the classes that are registered manually must be exposed to the bindings and to the class reference. Classes that are initialized on _post_initialize must be be registered in a special way so that bindings and class reference generators can differentiate them from the rest of the classes and ignore them.
• When the engine wants to pass an instance of one of those special classes to C#, we need to create a managed instance that wraps it, and we do that by searching the class name via reflection. PhysicsDirectSpaceStateSW won't work. We need either get_class_name() or a new virtual method to return the name of the closest registered parent class (PhysicsDirectSpaceState in the case of PhysicsDirectSpaceStateSW).

@reduz WDYT?