Hiding top-layer toolbars via JSON state

I'd like to embed neuroglancer on a website and to make things look nice, I'd like to hide the top-bar, in particular, setting the display property to `none` for the div elements `neuroglancer-viewer-top-row` and `neuroglancer-layer-panel` via an attribute in the JSON state.

Would this be a feature you'd want to merge into master?

I'm not familiar with how to extend the JSON state without breaking backwards compatibility, would you be able to guide me where to make changes?

Thanks.