Allow widgets to display more than once within container widgets.

Adds the ability to display multiple widgets within container widgets and adds indexing by view.id in addition to model.id.

closes #5958
Editing on my previous comment. It would be nice if the call to  `create_child_view` in this file (in function add_child_model) would use the value `{parent: this}` for the `options` argument, which would then give access to the parent in child widgets.