Fixes to address deprecations in the NIPagingScrollView component

These are some fixes to address recent deprecations/refactorings in the NIPagingScrollView component:
- [NIPagingScrollView pagingScrollView] was removed without deprecation in commit c6d0af854b1d1ca2897bf8d07a49ad3ae6f69fd8.
- NIPageView is deprecated and thus its implementation generates a warning.