Add Composite Child machinery and APIs to GtkWidget

This commit implements the needed machinery for GtkWidget
to build it's composite content from GtkBuilder XML and
adds the following API:

  o gtk_widget_init_template()

    An api to be called in instance initializers of any
    GtkWidget subclass that uses template XML to build it's components.

  o gtk_widget_class_set_template()

    API to associate GtkBuilder XML to a given GtkWidget subclass

  o gtk_widget_class_automate_child()

    API to declare an object built by GtkBuilder to be associated
    with an instance structure offset and automatically set.

 o gtk_widget_get_automated_child()

   API for bindings to fetch a child declared to be automated by
   gtk_widget_class_automate_child(), for the case where bindings
   do not generate GObjects under the hood and cannot use structure
   offsets to resolve composite object pointers.

 o gtk_widget_class_declare_callback[s]()

   Declare static functions to be used in signal callbacks from
   a given class's template XML

 o gtk_widget_class_set_connect_func()

   API for bindings to override the signal connection machinery
   for a given GtkWidget derived class.

gtk/gtkbuilder.h

@@ -36,7 +36,6 @@ G_BEGIN_DECLS
 
 #define GTK_BUILDER_ERROR                (gtk_builder_error_quark ())
 
-typedef struct _GtkBuilder        GtkBuilder;
 typedef struct _GtkBuilderClass   GtkBuilderClass;
 typedef struct _GtkBuilderPrivate GtkBuilderPrivate;
 
@@ -108,14 +107,6 @@ struct _GtkBuilderClass
   void (*_gtk_reserved8) (void);
 };
 
-typedef void (*GtkBuilderConnectFunc) (GtkBuilder    *builder,
-				       GObject       *object,
-				       const gchar   *signal_name,
-				       const gchar   *handler_name,
-				       GObject       *connect_object,
-				       GConnectFlags  flags,
-				       gpointer       user_data);
-
 GType        gtk_builder_get_type                (void) G_GNUC_CONST;
 GtkBuilder*  gtk_builder_new                     (void);