(bug 28868) Include the number of pages in the default getLongDesc for multipaged...

(bug 28868) Include the number of pages in the default getLongDesc for multipaged documents

This will only include the number of pages if there is at least two pages.
If you have a pdf with a single page, it won't say 1 page. I modeled this
on how animated images will only say x frames if x > 1.

(For reference, this will affect extensions like pdfHandler, but it won't
affect extensions like PagedTiffHandler since that one totally overrides
the default method)