Use a GtkHeaderBar for the credits and about buttons.
It makes less sense here than in other places to go back to
the buttons on the bottom, considering we only have a close
button, so we always use a header bar.
https://bugzilla.gnome.org/show_bug.cgi?id=720059
These files are generated, so adding them to git is somewhat
icky, but it helps translators who currently can't use intltool-update
on a fresh git checkout.