Widen the GTK askalg() message box.

In GTK3, the line 'Continue with connection?' got wrapped (in spite of
my attempt to enforce via string_width() that it didn't - probably a
few pixels were needed on top of that for various padding and
furniture) so it looked even sillier. But it looked a bit narrow to be
sensible even in GTK2, so the simplest answer is just to widen it
considerably.