CollectionView reset event should use _removeChildModels

[paulfalgout]

It can iterate over the reset event's previousModels This prevents a collection.reset from removing customly added views.

We should also indicate that child view was created from a collection model rather than using this._children.findByModel to account for views added from a collectionview that do not have the collection's model. We should add a custom index so that we can find children by the custom index. All of this can be private API.