Materials of imported scene should be collected under a single item under root object

When a complex 3D scene is imported, it may be difficult to find the material used by the model when the same material is used by multiple models. All of the materials of the scene (or at least materials used by multiple models) should be generated under a single materials item for easy discovery.

I.e. something like this:

Node {
    id: rootNode
	
    Node { // This doesn't need to be a Node, so maybe some other type is better?
        id: materials
        DefaultMaterial {
            id: body_material
            //...
        }
        DefaultMaterial {
            id: glass_material
            //...
        }
        //...
    }