read more metadata for poppler

"creator" tags have typically been mapped to the "author" property for KFileMetaData. Unfortunately, both tags can be present in PDFs. Since for whatever reason creator is one of the very few metadata tags that QPdfWriter supports, we should be able to read it. This is solved here by reading the creator tag when no author tag is found. Alternatively, a separate creator property must be added to KFileMetaData and quite a lot of extractors must be switched over.

Also extract the page count.