StWidgetAccessible: accessibility support for StWidget

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