[BUGFIX beta] CollectionView keeps child views and content in sync. Closes #4306

CollectionView.arrayWillChange and CollectionView.arrayDidChange were
getting called multiple times when content was an ArrayProxy with a
computed property ArrangedContent relying on its content.

This is probably not the best way to fix it and it breaks a
CollectionView test that counts the life cycle events fired when content
is modified.

It does get the job done, though, and doesn't seem to break
anything.