pdf: move PDFStructureWriter into own file, rename to COSWriter HEADmaster

This needs to duplicate some functions from pdfwriter_impl to make
this possible.

Change-Id: Ie738c9690306d6498c883f034a3fcd6483f24eef
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/176985
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/178762
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>