Upgrade gtk-builder-convert to newer version that adds useful warnings.

The notes on "Migrating from libglade to GtkBuilder" in the GTK+
Reference Manual at, e.g.
http://library.gnome.org/devel/gtk/unstable/gtk-migrating-GtkBuilder.html
say that "While libglade can often tolerate multiple widgets having the
same id in a glade file, GtkBuilder will not accept duplicate object ids."
The version of gtk-builder-convert that we had previously didn't warn about
duplicate ids; the new version checked in by this commit does.