alternate layer/group api

Sometimes the user may want to toggle layers within groups. This is not possible with the API at the moment that is geared toward either selecting entire groups or ungrouped layers.

We could consider an alternative API for `layer_names` that allowed exact specification e.g.:

```
list(
"group_name1" = c("layer_name1", "layer_name2"),
"group_name2" = "layer_name1"
)
```

etc.