* Add _ASCollectionReusableView container for ASCollectionNode supplementary nodes with applyLayoutAttributes support
• Adds support for -[ASCellNode applyLayoutAttributes:] to supplementary nodes
* Ensure _ASCollectionReusableView has layoutAttributes if the collection view doesn’t set them
* Use correct layoutAttributes accessor for supplementary elements
* Add unit test to test trait collection changes occur during updates are handles correctly
* Remove handling code in ASDataController:updateWithChangeSet. Previous test should fail
* Correctly handle trait collection changes that occur mid-updates
- Currently, when there is a new trait collection, we correctly propagate it to all visible elements. However, since the propagating block is executed on main thread immediately without waiting for the background editing queue of ASDataController, not all elements are updated.
- Then to fix that, we updated ASDataController to handle these changes inside updateWithChangeSet. This works, but it doesn’t address the underlying issue.
- We now delegate the propagating task to ASDataController which schedule a block to its main serial queue after waiting for its background editing queue.
