EPUB_SPLIT_METHOD is still hardcoded to HEADING, so while we send the
page break info to librevenge now, it's ignored on that end.

This requies basic infrastructure for automatic styles.

Change-Id: Ibafead0dedd9dbfa6223a9c701a62611ba2671fd
Reviewed-on: https://gerrit.libreoffice.org/41573Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>