Add new defgroup image-dired-external

Add new defgroup image-dired-external

* lisp/image/image-dired-external.el (image-dired-external): New
defgroup.
(image-dired-cmd-create-thumbnail-program)
(image-dired-cmd-create-thumbnail-options)
(image-dired-cmd-pngnq-program, image-dired-cmd-pngnq-options)
(image-dired-cmd-pngcrush-program)
(image-dired-cmd-pngcrush-options)
(image-dired-cmd-optipng-program)
(image-dired-cmd-optipng-options)
(image-dired-cmd-create-standard-thumbnail-options)
(image-dired-cmd-rotate-original-program)
(image-dired-cmd-rotate-original-options)
(image-dired-temp-rotate-image-file)
(image-dired-rotate-original-ask-before-overwrite)
(image-dired-cmd-write-exif-data-program)
(image-dired-cmd-write-exif-data-options): Use above new defgroup.