Make the headers-visible property readwrite instead of just writable, and

Make the headers-visible property readwrite instead of just writable, and

2005-01-13  Matthias Clasen <mclasen@redhat.com>

	* gtk/gtktreeview.c: (gtk_tree_view_class_init),
	(gtk_tree_view_set_headers_clickable): Make the headers-visible
	property readwrite instead of just writable, and remove the
	g_return_if_fail check that there is a model when setting this
	property. Also improve the blurb.  (#163851, Richard Hult)