image-dired: Support wrap-around in display-next-thumbnail-original

image-dired: Support wrap-around in display-next-thumbnail-original

* lisp/image-dired.el (image-dired-forward-image): Support negative
prefix argument.  Optionally wrap-around, which means to continue from
first image after reaching the last one and vice versa.
(image-dired-backward-image): Reimplement in terms of
'image-dired-forward-image'.
(image-dired-display-next-thumbnail-original)
(image-dired-display-previous-thumbnail-original): Wrap around; when
reaching the last image, continue from the first one.  Add optional
prefix argument to move more than one image at a time.