* Fixes issues with accessibility elements in scroll views
There are two changes here:
1. Because we can't reliably update the screen positions of accessibility elements
contained within a scroll view, we need to calculate them on demand, so we override
the accessibilityFrame method to do that.
2. Throwing away our calculated accessibility elements on frame change seemed to
cause the accessibility system to be confused.
Combining these two fixes together results in success, yay!
* Don't set the accessibilityFrame, getter is overridden.
* First approach to improve accessiblity
* Clear accessibleElements in addSubview: and willRemoveSubview:
* Adjust comments and rename viewNode to node
* Create new accessible elements if screen coordinates of view changes
* Remove legacy clearing of accessibleElements
* Performance improvements
* Use bounds for screenFrame calculation and indexOfObjectIdentical: in indexOfAccessiblityElement:
* Add ASDK_ACCESSIBILITY_DISABLE compiler flag to disable custom accessibility code in ASDK
* No need to set a frame if a subnode view is an accessibility element and not layer backed
- Calculate accessibilityFrame for accessibleElements already in accessibleElements method
- Remove asyncdisplaykit_node associated object as it's not needed anymore
- 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:
