As the result of our discussion with Risa Wolf and Ray Lee about default titles for audiobook spine elements should look like `[_TOC name_|"Chapter x"] ([file x of y])`.

For example, the first of 10 spine elements with title "Introduction" should be displayed: 

`Introduction (file 1 of 10)`

the second of ten named "Part 1" should be visible as:

`Part 1 (file 2 of 10)`

the first spine element of 10 without a title will be shown as:

`Chapter 1 (file 1 of 10)`

The word we use to name spine elements in audiobook when they has no title is "Chapter"