make the datalists a safe type (not using a generic gpointer) by expecting

make the datalists a safe type (not using a generic gpointer) by expecting

Fri Sep 18 18:46:14 1998  Tim Janik  <timj@gtk.org>

        * glib.h:
        * gdataset.c: make the datalists a safe type (not using a generic
        gpointer) by expecting a GData* argument in the g_datalist functions.
        provide g_dataset_foreach() and g_datalist_foreach() functions that
        allow a GDataForeachFunc function to walk the data lists.
        (g_dataset_destroy_internal): made this function truely reentrant (i.e.
        can be called from within destroy notifiers as well).
        the *_foreach functions are _not_ reentrant (unless all the other
        dataset and datalist functions).