Allow page-numbering-start-at key in theme for prepress book to reference page before body

[mojavelinux]

In a prepress book, the first page of the body is blank. If the value of the page-numbering-start-at key in the theme is 0, begin the page numbering on this blank page so the preamble and chapter pages start on even number pages.

[mojavelinux]

Another possibility to look into is to treat a numeric value literally, which would be the 1-based index starting on the blank page before the first content page of the body. The symbolic value "body" would retain the existing behavior.

[mojavelinux]

It turns out we can use the value 1 for this since the value 1 wasn't working anyway. See #2255.

The symbolic value "body" will still refer to the page after the empty verso page. That will avoid breaking any existing documents.

[mojavelinux]

After giving this more thought, I'm back to the thought that the value for this should be 0 instead of 1. The reasoning is that we are treating page 1 of the body to be the first page of content. And since 0 doesn't have any other meaning in this context, it serves to communicate that we want the page before that page to be where the page numbering / running content starts. The "0" page.

[mojavelinux]

Btw, this issue pertains to both the page numbering (page-numbering-start-at) and the running content (running-content-start-at).