Extract expandChild a to VivideObject

Nobody has claimed this yet.

`expandChild` asynchronously fetches the child layer of a `VivideObject`.

Having a `VivideView` dynamically setting the `expandChild` method onto a widget, seems counterintuitive.

Instead, this method should be exposed by the `VivideObject` itself:

```JavaScript
childLayer = await vivideObject.getChildren();
```

---

Also, a widget should not be in charge of setting the `childLayer` of a `VivideObject`:

```JavaScript
object.childLayer = await this.expandChild(childData, childLayer.script);
```

Locate the VivideObject, VivideView, widget, and expandChild entry points, then trace how child layers are currently fetched and assigned. Define the change around VivideObject.getChildren() so widgets no longer install expandChild or assign childLayer themselves, and verify all existing callers still obtain the expected child layer.

Written by the indexing model from the issue text.