Scenario: An ASCollectionNode is a subnode of an ASCellNode. A layout transition is started, resulting in
the removal of the ASCollectionNode as a subnode. As it is removed, the hierarchy state is cleared - including
the "range managed" bit - on the ASCollectionNode. However, the deep recursion traverses the layer hierarchy
too, and clears this bit on the ASCellNodes inside the ASCollectionNode. A moment later, the collection performs
its final ASRangeController update to mark its cells as invisible and free memory. Then an assertion is triggered
in ASRangeController, because it is operating on nodes that do not have the "range managed" bit set.
It turns out that ASInterfaceState also propogates in this way, but that behavior is efficient and beneficial in
its current configuration (it assists how multi-dimensional preloading works). However, hierarchy state should
never need to jump discontinuities in the node hierarchy. For now, disabling that case and will revisit
other use cases soon.
- Add class method to create a UIAccessibilityElement from a ASDisplayNode
- Add function to iterate through a ASDisplayNode tree in bfs fashion
- Add assert for _accessibleElements in accessibilityElementAtIndex:
ASDisplayNodes now have an overidable method -placeholderImage that lets you provide a custom UIImage to display while a node is displaying asyncronously. The default implementation of this method returns nil and thus does nothing. A provided example project also demonstrates using the placeholder API.
Summary:
* Moves ASThread.h from Private to Public visibility
* Moves ASThread.h from Private/ to Details/
* Changes public #imports from "" to <>
Test Plan:
```
#import <AsyncDisplayKit/ASDisplayNode+Subclasses.h>
#import <AsyncDisplayKit/ASControlNode+Subclasses.h>
```
