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.

====

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.
I went straight to the code and missed the comments in the conversation section.

This is looking very clear. I'm looking forward to have this event merged. My only concern is that it will make the `show` method too heavy for slow devices, with `getNestedViews` calling itself dozens of times with even being used. For this event I feel it should be optionally triggered, passing an option to `show`:

```javascript
if (view.triggerAttach && !!showOptions.triggerAttach) { ... }
```