Restore Build-Depends: dh-sequence-gnome: used by Ubuntu

@smcv Ubuntu's gnome-pkg-tools runs dh_translations along with dh_gnome to prepare the translations for Ubuntu's language packs. Could we add dh-sequence-gnome back to the packages we have dropped it from?

Specifically, we need it for packages in Ubuntu main, in addition to other packages where X-Ubuntu-Use-Langpack: yes is set. (But we will probably be dropping X-Ubuntu-Use-Langpack: yes from many of the packages.)