* maps-2.0, maps-2.4: Register GtkCellEditable, GtkTreeModel,
	GtkTreeSortable and GtkFileChooser as GInterfaces since that's
	what they are.

	* xs/GtkTreeModelSort.xs: Make the xsubs for
	gtk_tree_model_sort_new_with_model and gtk_tree_model_sort_new
	return GtkTreeModelSorts instead of GtkTreeModels.  The latter is
	an interface and therefore abstract.

	* t/GtkCellView.t: Remove redundant isa_ok test.

svn path=/trunk/; revision=1611