Wrap, rename, and delegate NavigationTitleScrollView.

Wrapping eventually allows masking and separating
component and scroll page widths. The wrapper is
called NavigationTitlePickerView.

Because of wrapping, KVO doesn't work anymore.
Instead of adjusting by chaining KVO, just use
delegation, which is more conventional anyway.
Add a debug_view helper for visually checking frames.
Minor section comments.
Redo NavigationTitlePickerView masking.

Scroll views cannot be masked; the mask gets
applied to their contents. They have to be wrapped
in another view.