Improve support for GtkBuilder 4.
Reported by Stuart <stuart.a.hayhurst@gmail.com> in
<https://lists.gnu.org/archive/html/bug-gettext/2023-07/msg00010.html>.
* gettext-tools/its/gtkbuilder.its: Recognize translatable="1" and similar as
equivalent to translatable="yes".
* NEWS: Mention it.
