Added support for [will/did]TransitionFromLayout:toLayout:

- This needs to be checked by someone more familiar with the code base. I haven't studied it long enough to know if this is the best place to send these messages to the cell. Also, I'm not convinced it happens at the same time as when it does for iOS' UICollectionView. However, this worked for my purposes so I'd like to get something in the code base to properly call these methods. :question: