notifications: Drop the dep on libgnome-menu

This removes a leftover configure check on libgnome-menu-3.0. The gmenu
APIs were only ever used in the WIP notification panel patches in
https://bugzilla.gnome.org/show_bug.cgi?id=685928 , but the final code
that landed used equivalent gio API instead.
[l10n] Updated Catalan (Valencian) translation