Adds onAttach event.

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.
Actually, I don't think it's necessary in this case. `onDomRefresh` doesn't have one, either, and these are the one type of action that I think it's not necessary to have a `before`. If you want to do something before it's attached, then there's `onRender` and such.