image-dired: Rewrite and extend slideshow feature

image-dired: Rewrite and extend slideshow feature

* lisp/image/image-dired.el
(image-dired--slideshow-start-timer)
(image-dired--slideshow-stop-timer)
(image-dired--slideshow-show-message): New functions.
(image-dired--slideshow-current-delay): New variable.
(image-dired--slideshow-initial): Delete variable.
(image-dired-slideshow-start): Simplify and ensure we display the
image at start.
* lisp/image/image-dired.el (image-dired--slideshow-stop): Add support
for pausing, and going backwards and forwards during slideshow.