imshow.m: Add support for 'parent' property (bug #45473). * imshow.m: Add cell variable prop_val_args{:} when calling image or imagesc. Populate prop_val_args with {"parent", hparent} when "parent" argument given to imshow. Remove code commented out in 2015/05/1 which has not caused any problems. Add warning that arguments "border" and "reduce" are not supported.