Fix some image-dired customization types

Fix some image-dired customization types

* lisp/image-dired.el (image-dired): Add info link to defgroup.
(image-dired-dir, image-dired-gallery-dir): Set type to directory.
(image-dired-db-file, image-dired-temp-image-file):
(image-dired-cmd-create-thumbnail-program):
(image-dired-cmd-create-temp-image-program):
(image-dired-cmd-rotate-thumbnail-program):
(image-dired-cmd-rotate-original-program):
(image-dired-temp-rotate-image-file):
(image-dired-cmd-write-exif-data-program):
(image-dired-cmd-read-exif-data-program): Set type to file.
(image-dired-create-thumb, image-dired-line-up-dynamic): Check storage
type at runtime, since setting image-dired-thumb-size does not
automatically set image-dired-thumb-width and image-dired-thumb-height.