Adds a bookmark which points to the current page.
Bookmarks are specified as paths in much the same way as file
paths. Headings and subheadings are separated by a backslash
character. For example:
"1. Introduction\1.1 Aim and Methods\1.1.3
When a bookmark is added, intermediate headings and subheadings
will be created if they do not already exist.
To add multiple bookmarks all referring to the same page, simply
call the AddBookmark method multiple times.
This method returns the Object ID of the newly added Bookmark