Add Composite Child machinery and APIs to GtkWidget

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.