Discern between virtual and abstract class bindings

* Previous "virtual" classes (which can't be instantiated) are not corretly named "abstract".
* Added a new "virtual" category for classes, they can't be instantiated from the editor, but can be inherited from script and extensions.
* Converted a large amount of classes from "abstract" to "virtual" where it makes sense.

Most classes that make sense have been converted. Missing:

* Physics servers
* VideoStream
* Script* classes.

which will go in a separate PR due to the complexity involved.

scene/animation/root_motion_view.h

@@ -71,7 +71,6 @@ public:
 	bool get_zero_y() const;
 
 	virtual AABB get_aabb() const override;
-	virtual Vector<Face3> get_faces(uint32_t p_usage_flags) const override;
 
 	RootMotionView();
 	~RootMotionView();