Before writerperfect::exp::XMLImport grows into a God object. Also, if
I'm at it, recurse into text:h to include heading text.

Change-Id: Iede56d00dbede40186b6484d0be92a2ac2e4768e
Reviewed-on: https://gerrit.libreoffice.org/41005Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>