Add tools to debug the layer dm

As the layer dm provides a unified way to handle all widgets, it would need a visual and string stream debugger (for agents).

The overall goal is to provide ways to:

- Inspect the current state of the different views / pipelines
- Activate / Deactivate pipelines from the debugger
- Inspect the actors present in the pipelines & VTK properties (-> introduce a GetManagedProps() -> std::vector API)
- Trace the interactions (interaction priorities / last focused / widget states)
- Force add a pipeline or manually trigger the creation (-> debug creators)
- Inspect the current state of the renderers / display renderers in isolation
- Provide helpers / patterns to unit test the pipelines / creators / widgets and interactions
- Inspect the custom cameras / shift them

The tool should be API driven first and provide:

- A scripted extension UI
- A trame-slicer UI

@RafaelPalomar if you have other suggestions, I would be happy to add them to this list!

Start by mapping the layer dm architecture, including its views, pipelines, actors, creators, widgets, renderers, and custom cameras. Review the proposed GetManagedProps() API and the API-driven debugger goals before narrowing the work into a concrete first scope; done requires an agreed debugging interface rather than the full list of requested capabilities.

Written by the indexing model from the issue text.