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.

## System-wide startup file for Octave.
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## This file should contain any commands that should be executed each
## time Octave starts for every user at this site.