image-dired: Navigate from image display buffer

image-dired: Navigate from image display buffer

* lisp/image-dired.el
(image-dired-mark-thumb-original-file)
(image-dired-unmark-thumb-original-file)
(image-dired-flag-thumb-original-file)
(image-dired-toggle-mark-thumb-original-file)
(image-dired-unmark-all-marks)
(image-dired-display-next-thumbnail-original)
(image-dired-display-previous-thumbnail-original): Make commands
usable from image-dired-display-image-mode.
(image-dired-display-image-mode-map): Bind above commands.
(image-dired--with-thumbnail-buffer): New macro.