StWidgetAccessible: accessibility support for StWidget

It includes:

 * Expose a proper focusable state, instead of the default one from
   cally, using StWidget::can_focus, and also notifying the state change

 * Management of the selected stated, using the current pseudo_class.

 * Defines a new virtual method on StWidget: get_accessible_type. In
   this way it is not required to reimplement get_accessible just for
   a accessible type change. get_accessible is reimplemented using this.
   You can see that as a substitute of the atk object factory

https://bugzilla.gnome.org/show_bug.cgi?id=636716

src/st/st-widget.h

@@ -82,6 +82,8 @@ struct _StWidgetClass
   gboolean (* navigate_focus)  (StWidget         *self,
                                 ClutterActor     *from,
                                 GtkDirectionType  direction);
+
+  GType    (*get_accessible_type)   (void);
 };
 
 GType st_widget_get_type (void) G_GNUC_CONST;