one-shot interface initializers

[Tim Janik <timj gtk org>]

owen and me have been doing some pondering on irc about the current
signals-for-interfaces situation, where people basically have to do:

static guint base_counter = 0;
void interface_base_init (Iface *iface)
{
  if (base_counter++ == 0)
    {
      iface->sig_id = g_signal_new ("some-iface_signal", TYPE_IFACE);
    }
  else
    {
      iface->sig_id = g_signal_lookup ("some-iface-signal", TYPE_IFACE);
    }
}
void interface_base_finalize (IFace *iface)
{
  base_counter--;
  if (base_counter == 0)
    _g_signals_destroy (TYPE_IFACE);
}

things can easily be changed to eliminate the need to call
_g_signals_destroy() at all, and the base_counter/interface_base_init()
"hack" could be changed to read:

void interface_init (gpointer dummy_pointer,
                     gpointer class_data)
{
  g_signal_new ("some-iface_signal", TYPE_IFACE);
}
void interface_base_init (Iface *iface)
{
  iface->sig_id = g_signal_lookup ("some-iface-signal", TYPE_IFACE);
}

by setting interface_init() and it's second argument class_data as
the class_init and class_data arguments in the interfaces GTypeInfo
structure.

there're two things to consider here, for normal classes,
base_init(class) is called prior to class_init(class, class_data).
for interfaces, class_init(NULL, class_data) would be called prior
to base_init(iface), and the class_init()'s "class" argument
would actually be a dummy pointer of NULL, as it's going to
be called _prior_ to creations of any interfaces of this type.

i'd apprechiate comments from especially jrb and bill, as i
remember both expressing concern about the current need for
a base_counter type hack for creating signals of interfaces.

---
ciaoTJ