Request to support access geometry's attributes in Open3DScene, and request to support access mouse position in pointcloud coordinate in gui scene.

### 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

1.Request to support access geometry's attributes in Open3DScene
In Class Open3DScene, we can add_geometry,but there is not interfae or attribute to access the geometry that we added through "add_geometry".
2.Request to support access mouse position in pointcloud coordinate?
In gui, we can use set_on_mouse to get an mouse event, which contain the position in attirbutes "x" and "y".
Just like this ,will your team support access position in pointcloud coordiante.

### References

_No response_

### Additional information

_No response_