added gtk_object_set_data_destroy and gtk_object_set_data_destroy_by_id

added gtk_object_set_data_destroy and gtk_object_set_data_destroy_by_id

Thu Sep 17 05:12:48 1998  Tim Janik  <timj@gtk.org>

        * gtk/gtkobject.h: added gtk_object_set_data_destroy and
        gtk_object_set_data_destroy_by_id functions to set the destroy
        function pointer for an existing data portion.

        * gtk/gtkobject.c: removed the GtkObjectData implementation and
        made all gtk_object_*data* functions use the GLib equivalents.