Add VisualShader node groups

- Add `VisualShaderGroup` ressource
- Introduce new ShaderGraph abstraction used by both `VisualShader` and
  `VisualShaderGroup`
- Add `VisualShaderGroupNode` which hosts a group, plus
  `VisualShaderNodeGroupInput` and `VisualShaderNodeGroupOutput`
- Refactor and extend the visual shader editor to allow editing
  `VisualShader`s and `VisualShaderGroup`s (arbitrarily nested) in a
  unified way.