Adds _getNestedView to Layout and Collection Views.

Part 2 of adding [onAttach](https://github.com/marionettejs/backbone.marionette/pull/1886). This adds `_getNestedViews` to LayoutView and CollectionView.

@jasonLaster made a good point that they should only be used when creating new abstractions, so I opted for making them private.

These aren't used now, but the third and final PR will add the `onAttach` event that *will* use this.
I think it makes more sense to be in a view, actually. The view can have nested children whether it's in a region or not. Also, collecting the nested children is different depending on whether it's a LayoutView or a CollectionView. It's weird to put, say, babysitter logic in anything other than a CollectionView/CompositeView, I think.