initializing WidgetView.child_views as object

Based on [how it is used](https://github.com/ipython/ipython/search?q=child_views&ref=cmdform&type=Code), `child_views` should just be an object.

Specifically, I ran into this while trying to extend `ContainerView`, and needed a list of its contents. underscore freaks out and won't let you use any of its Collection methods (which could pull out the id: value pairs) in a meaningful way.