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
Some of the labels were not marked as no-show-all. But the
code clearly tries to manage their visibility, so gtk_widget_show_all()
should not affect them.
https://bugzilla.gnome.org/show_bug.cgi?id=681484