It doesn't do anything yet, but EPUBExportFilter::filter() is called.
Actual functionality will be provided by libepubgen later.

Change-Id: I7229dc33c69238c6e98a7bb30fd38473b3c35bfe
Reviewed-on: https://gerrit.libreoffice.org/40630Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>