Also enhance RigidBody docs as per https://github.com/godotengine/godot-docs/pull/1018 and fix the version tag in all files (not really stable yet, but it makes no sense to hardcode rc3 at this stage).
This makes a VisualScriptEditor singleton, which gives plugins the ability to register their own custom nodes. Those will be available for insertion in the Visual Script editor, under the "Custom Nodes" category.
