image-dired: New defgroup and section for gallery support

image-dired: New defgroup and section for gallery support

* lisp/image-dired.el: Add new section header "Gallery support" to
improve file organization.  Move relevant TODO:s there.

(image-dired-gallery): New defgroup.
(image-dired-gallery-dir, image-dired-gallery-image-root-url)
(image-dired-gallery-thumb-image-root-url)
(image-dired-gallery-hidden-tags): Add options to above new defgroup.

(image-dired-add-to-file-comment-list)
(image-dired-create-gallery-lists)
(image-dired-add-to-tag-file-lists, image-dired-hidden-p): Rename to...
(image-dired--add-to-file-comment-list)
(image-dired--create-gallery-lists)
(image-dired--add-to-tag-file-lists, image-dired--hidden-p):
...this, to make it more clear that these functions are internal.
Make the old names obsolete function aliases.
(image-dired-add-to-tag-file-list): Make unused function obsolete.