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.

docs/reference/gtk/gtk3-sections.txt

@@ -5385,6 +5385,18 @@ gtk_widget_set_vexpand_set
 gtk_widget_queue_compute_expand
 gtk_widget_compute_expand
 
+<SUBSECTION Templates>
+gtk_widget_init_template
+gtk_widget_class_set_template
+gtk_widget_class_set_template_from_resource
+gtk_widget_get_automated_child
+gtk_widget_class_bind_child
+gtk_widget_class_bind_child_internal
+gtk_widget_class_automate_child
+gtk_widget_class_bind_callback
+gtk_widget_class_declare_callback
+gtk_widget_class_set_connect_func
+
 <SUBSECTION Standard>
 GTK_WIDGET
 GTK_IS_WIDGET