feat: Layer descriptions

Is there any way to store a description of each layer in the JSON state? It would be very helpful if I could display a ~1 sentence explanation for some of the layers in my neuroglancer scene (without necessitating in a super long layer `name`).

I guess in the JSON state it would look like this:

```json
{
"layers": [
{
"name": "my objects",
"description": "Object detections after filtering out those smaller than 1 um^3",
...
}
]
}
```

I don't have a strong opinion about where to actually display the description. Maybe as a tooltip in the layer list? Or via a new header button in the layer control panel, perhaps named `?` or `i`?