By switching to HtmlWriter instead of manually constructing HTML
elements / attributes.

Also fix a validity problem for ReqIF (transitional XHTML still allows
that, so not depending on mbXHTML here).

Change-Id: I4f3fef781fdbb4c72824701ad6d380cece8e6f81
Reviewed-on: https://gerrit.libreoffice.org/53120Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>