Replace PagingOptions with property assignments

[rechsteiner]

This introduces a breaking change: All configuration is now moved
to properties on the PagingViewController class instead of having to
pass in an instance conforming to the PagingOptions protocol.

This reduces the boilerplate of having to create a separate options
struct when you just need to override a single value. It also means
you can change the options after the PagingViewController has been
initialized. Lastly, this change also means we can introduce better
support for customizations through Storyboard in the future.

Before:

struct Theme: PagingTheme {
  let textColor: UIColor = .red
}

struct Options: PagingOptions {
  let theme: PagingTheme = Theme()
  let menuItemSize: PagingMenuItemSize = .fixed(width: 100, height: 40)
}

let pagingViewController = PagingViewController(options: Options())

After:

let pagingViewController = PagingViewController()
pagingViewController.menuItemSize = .fixed(width: 100, height: 40)
pagingViewController.textColor = .red