Added GProxy interface for proxy extension point

Implement an extension point for proxy protocol implementation. This
is mainly useful for socket-based proxy where it is possible to use the
proxied socket the same way it would for other stream based socket.

Reviewed-by: Dan Winship <danw@gnome.org>

gio/giotypes.h

@@ -206,6 +206,7 @@ typedef struct _GVfs                          GVfs; /* Dummy typedef */
  * Since: 2.26
  **/
 typedef struct _GProxyResolver                GProxyResolver;
+typedef struct _GProxy			      GProxy;
 typedef struct _GProxyAddress		      GProxyAddress;
 
 /**