Improve component appearance and disappearance events #559

Appearance/disappearance events can be inconsistent and unreliable, likely due to the fact they are typically driven by the underlying UICollectionView. We should investigate better alternatives that satisfy the following properties:

1. Components that are added while a tree is onscreen should receive a -[CKComponentController componentTreeWillAppear] event
2. Components that are removed while a tree is onscreen should receive a -[CKComponentController componentTreeDidDisappear] event
3. There should be a guaranteed balance between both events; a-[CKComponentController componentTreeWillAppear] event should be followed by at most one -[CKComponentController componentTreeDidDisappear] event