Small string freeze break in GTK+

[Matthias Clasen <matthias clasen gmail com>]

I've committed a tiny string addition in GTK+, by making GtkBuilder
report a meaningful error message when it encounters a duplicate id. I
think this is justified, since it will help a lot with the upcoming
effort to move from libglade to GtkBuilder.

The new string is "Duplicate object id '%s' on line %d (previously on line %d)".


Matthias