new Layer() with existing definition results in sublayer id's being identical

when i create a new artboard from an existing artboard:

` const artboard = new Artboard(source_artboard);
`

only the outer layer ID is created anew; all sublayers are identical, resulting in interesting behavior within Sketch: I select a sublayer, and both that sublayer and the duplicated object's sublayer are selected.

My current workaround is creating a newObject function that will create new object Ids for each sublayer:

```
let newObject = source_obj => {
Object.keys(source_obj).forEach(key => {
if (key === "do_objectID") {
source_obj[key] = uuidv1().toUpperCase();
return source_obj;
}
if (typeof source_obj[key] === "object") {
newObject(source_obj[key]);
}
});
return source_obj;
};
```

and instead call it

` const artboard = newObject(source_artboard);
`

It'd be nice if new Artboard(source_json) would do the same. Any reason not to?

Start by tracing the Artboard constructor with an existing artboard definition and inspect how nested objects retain their do_objectID values. Compare the resulting object tree with the recursive workaround shown in the issue; done means duplicated artboards and all sublayers have distinct IDs without requiring callers to rewrite the object.

Written by the indexing model from the issue text.