This was another piece of code that determined text size by
instantiating a GtkLabel and asking for its size, which I had to fix
in gtkfont.c recently because that strategy doesn't work in GTK3.
Replaced the implementation of string_width() with a call to the
function I added in gtkfont.c, and now dialog boxes which depend on
that for their width measurement (e.g. the one in reallyclose()) don't
come out in silly sizes on GTK3 any more.
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