Extend vcl::PDFExtOutDevData so that it's possible to use it outside the
PDF export; this way the EPUB export can know which chapters start on
which page.

This means fixed and reflowable layout has the same table of contents,
instead of just Page <N> in the fixed layout case.

Change-Id: I935fb23c66ec747431b91e83b0e677d4e5f704b9
Reviewed-on: https://gerrit.libreoffice.org/48332Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>