[2.1] Backport FOPNGSerializer

Cocoon 2.1 still don't have official support for FOP 0.9x, while Cocoon 2.2 does

This compatibility is only a matter of FOPSerializer (called FOPNGSerializer in the Cocoon 2.2 fop-ng block)

I won't put a patch here because it would only be a copy of the FOPNGSerializer.java from Cocoon-2.2, but it would be great if someone could backport the fop-ng block