Adds onAttach event.

#### This PR is being split up

To make this easier to review, I'm splitting it up into three bite-sized chunks.

– #1946 - Adds isNodeAttached helper
– #1949 - Adds _getNestedView
– TBA - Adds triggerAttached triggerMethod

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Mmk, this is ready for review.

- Works properly for infinitely nested LayoutViews
- Also works on collectionViews
- Documented
- Backwards compatible

This adds `onAttach`. Ref #1856 for more about why this event is important.

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To do: It adds three new public methods that need documentation

- triggerAttach (view). This should prob. just be private
- getNestedViews (layoutView)
- isNodeAttached

I'll do this once I get thumbs on the implementation.