Automatically number book parts (rather than as part of their title)

[wenkokke]

Currently, the numbers of the book parts are hardcoded as part of their titles. This won’t play well with formats which automatically number sections, such as LaTeX and EPUBs, and is prone to errors.

It would be better to automatically number these parts based on their position in the table of contents, and on whether or not they are part of the main matter… or at the very least extract the numbers from the titles and store them separately as part of the table of contents, so that different build paths can choose whether or not to use them.