Minor renaming to NavigationTitlePickerViewDelegate.

Naming should reflect the TitleScrollView as the
main class.
Refactor NavigationTitleScrollView to use a data source.

This makes it less awkward when adding the title
items. It also allows for future optimizations
like subview reuse pools.
Refine Swift usage and fixup weak property declarations.
Cleanup KVO debris for NavigationTitleScrollView.