Name new bookmarks by page label rather than number

New bookmarks are currently named by one-indexed page number. Using page labels is preferable in order to line up with everything else the user is shown (the TOC, page numberings in the document, etc.).

The only notable thing about the implementation is that the first commit removes some code duplication so that the new naming logic is only needed in one place (I've hopefully made sure the behaviour is identical though).

Thanks.