Use accessor functions instead direct access (GSEAL work)

Still remaining:
GtkAccessible::widget
GtkAssistant::forward
GtkAssistant::back
GtkObject::flags
GtkTreeStore::stamp

The GtkAssistant fields are related to bug #596428.  We don't
need accessor functions so much as the enhancement described
there implemented.

https://bugzilla.gnome.org/show_bug.cgi?id=615613