Operator panel: Enable editing 3D Viewer elements and test simulation
Problem
- Model elements from the "3D Viewer Objects" and the "3D Viewer Scene" categories are shown only in expert model in "Model Element Library"
- When expert mode is activated, it is currently not possible to add "sub-elements" to the "3D Viewer"
Analysis:
- Since the respective compositors are not implemented
- A generic new menu needs to registered for the "3D Viewer" editor
Expected behavior
- Elements are offered also in non-expert mode and can actually be added
- Simulation behavior elements is verified, e.g. by extended the "operator_panel_tutorial" from the file menu