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.c

@@ -216,6 +216,10 @@
  * <link linkend="GtkFileFilter-BUILDER-UI">GtkFileFilter</link>,
  * <link linkend="GtkTextTagTable-BUILDER-UI">GtkTextTagTable</link>.
  * </para>
+ * <para>
+ * Additionally, since 3.10 a special &lt;template&gt; tag has been added to the format
+ * allowing one to <link linkend="GtkWidget-BUILDER-TEMPLATES">define a widget class's components</link>.
+ * </para>
  * </refsect2>
  * <refsect2>
  * <title>Embedding other XML</title>