Graph editor - "Add" menu tweak

[Emmanuel Pacaud <emmanuel pacaud univ-poitiers fr>]

Hi,

I've just commited to goffice trunk a small improvement (I hope) to the
add menu, that should help to discover the numerous chart features
gnumeric provides.

This menu is now populated by all item that can be added unambiguously
to the selected chart.

I'd like to have some feedback on this change, so please give it a try
and tell me if anything is wrong.

Thanks,

        Emmanuel.