* gtk/gtkliststore.c, gtktreednd.c, gtktreestore.c:
Actually implement the GtkTreeDragSource::row_draggable virtual
function in GtkListStore and GtkTreeStore instead of just checking
whether it's implemented at all. This means that DnD isn't broken by
gtkmm's virtual function wrappers. The alternative would be to
hard-code the TRUE return value into gtkmm's wrappers, but that's
part of GTK+'s implementation, not it's API.