plug-ins: make file_gif_spin_button_int_init() generic.

Use the `value_pointer` parameter instead of modifying the globale
`gsvals.default_delay`. This doesn't change anything in current code
where file_gif_spin_button_int_init() is only used for connecting this
variable to the delay spin button, but could have been a problem if it
were reused in the future.

plug-ins/common/file-gif-save.c

@@ -1145,7 +1145,7 @@ file_gif_spin_button_int_init (GtkBuilder  *builder,
   gtk_adjustment_set_value (adjustment, initial_value);
   g_signal_connect (adjustment, "value-changed",
                     G_CALLBACK (gimp_int_adjustment_update),
-                    &gsvals.default_delay);
+                    value_pointer);
 
   return spin_button;
 }