Expose the object hierarchy to API

• Given a 3D asset in QML as a Mesh object,
• When I create a new Mesh with a model-tree based on the trunk of that asset, except with sub-branches stripped from it (by supplying a formatted string that uniquely identifies the root nodes of those sub-branches).
• Then a Mesh with a new scene-graph containing a copy of the original 3D asset without those sub-branches, is returned.

Developer notes:

I wish to be able to specify something like this:

    Mesh {
            id: model
            source: "bike.3ds"
    }
    Item3D {
            mesh: model
    }
    Item3D {
            mesh: model.get(bike.body.wheel1)
            x: -5
            z: 5
    }

This will help to resolve outstanding issues of corner cases fore model breakdown being inaccessible to users, as well as improve memory performance (currently we would potentially need to load a model twice - once for items which use the "complete" model, and once for the broken down sub-branch).

SP: 3