image-dired: Revamp slideshow functionality

image-dired: Revamp slideshow functionality

* lisp/image-dired.el
(image-dired-slideshow-delay): New defcustom.
(image-dired--slideshow-initial): New defvar.
(image-dired-slideshow-start): Don't show any prompts when starting a
slideshow, unless user gave a negative prefix argument.  Use the value
of the above new defcustom as the default delay.
(image-dired-slideshow-stop): Don't count number of images.  Instead,
continue the slideshow until the next command.
(image-dired-slideshow-step): Use the correct buffer.
(image-dired-slideshow-count, image-dired-slideshow-times): Make
obsolete.
(image-dired--slideshow-timer): Rename from
'image-dired-slideshow-timer'.  Make the old name into an obsolete
variable alias.
(image-dired-display-image-mode-map)
(image-dired-thumbnail-mode-map): Bind 'image-dired-slideshow-start'
to "S".
(image-dired-thumbnail-mode-menu): Add 'image-dired-slideshow-start'.