Replace image-dired-setup-dired-keybindings with a minor mode

Replace image-dired-setup-dired-keybindings with a minor mode

* lisp/image-dired.el (image-dired-thumbnail-mode): Fix docstring to
remove mention of nonexistent image-dired-dired and to refer to the
new minor mode.
(image-dired-minor-mode-map): New keymap assimilated from
image-dired-setup-dired-keybindings.  In the future, the keymap parent
should be removed, and perhaps also the duplicate bindings that
already exist in dired-mode-map.
(image-dired-setup-dired-keybindings): Remove.  Replace with an
obsolete function alias.
(image-dired-minor-mode): New minor mode, assuming the role of
image-dired-setup-dired-keybindings.