Request to change objects_ in O3DVisualizer to be saved in map

### 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).

### Proposed new feature or change

There are a few members in `O3DVisualizer::Impl` stored as vector, and calls to remove, show specific geometry needs to loop over the vector to match name. This can be slow when number of geometries is large, and this search can be called even at every animation frame.

We should consider either change the container to a map, or expose a writable GetGeometry interface so that user can implement their own data structure to speed up this search for some use cases.

https://github.com/isl-org/Open3D/blob/master/cpp/open3d/visualization/visualizer/O3DVisualizer.cpp#L303

https://github.com/isl-org/Open3D/blob/master/cpp/open3d/visualization/visualizer/O3DVisualizer.cpp#L1056

### References

_No response_

### Additional information

_No response_