How to convert LineSet to a Mesh

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

I am trying to visualize an object from which I would like to select some points. This object is within a dictionary containing a Mesh and a LineSet. The Mesh is divided in two meshes that are overlaped just by adding one to the other, while the LineSet is defining the boundaries of the area in which these two objects are located. However, the LineSet neither can be added to the Mesh nor can be added as a geometry, since the "vis.add_geometry()" function from "vis = o3d.visualization.VisualizerWithVertexSelection()" only allows one geometry per run. Is there a way to get the view of both LineSet and Mesh, while being able to select the vertex of the Mesh?