Add an event signal to GSocketListener

This allows the caller to know when a socket has been bound so that
it can for instance set the SO_SENDBUF and SO_RECVBUF socket options
before listen is called

https://bugzilla.gnome.org/show_bug.cgi?id=738207

gio/gsocketlistener.h

@@ -61,8 +61,11 @@ struct _GSocketListenerClass
 
   void (* changed) (GSocketListener *listener);
 
+  void (* event) (GSocketListener      *listener,
+                  GSocketListenerEvent *event,
+                  GSocket              *socket);
+
   /* Padding for future expansion */
-  void (*_g_reserved1) (void);
   void (*_g_reserved2) (void);
   void (*_g_reserved3) (void);
   void (*_g_reserved4) (void);