[SceneObject] Abstracted a generic Node class.
[Node] Bugfixing and better encapsulation

	* Fixed bug: Set parent doesn't remove previous parent relationship.
	* Removed GetChildren and added GetChildAt() for better encapsulation.
[Tree] Added abstract Tree class.
[SceneObject] Removed unnecessary this-> usage.
[SceneGraph] Abstracted tree functionality using a Simplex::Core::Tree instance.
[Readme] Updated TODO.