image-dired: Shorten several long names

image-dired: Shorten several long names

* lisp/image/image-dired.el (image-dired-image-mode-map)
(image-dired-image-mode): Rename from
'image-dired-display-image-mode-map' and
'image-dired-display-image-mode'.  Update all uses and make old
names into obsolete aliases.
(image-dired-display-this, image-dired-display-next)
(image-dired-display-previous): Rename from
'image-dired-display-thumbnail-original-image',
'image-dired-display-next-thumbnail-original', and
'image-dired-display-previous-thumbnail-original'.  Update all uses
and make old names into obsolete aliases.
* doc/emacs/dired.texi (Image-Dired): Update documentation for the
above changes, and improve indexing.