Support switching to hexl-mode from image-mode

Support switching to hexl-mode from image-mode

* lisp/image-mode.el (image-toggle-hex-display)
(image-mode-to-text): New functions.
(image-mode-map, image-minor-mode-map): Bind "C-c C-x" to
image-toggle-hex-display.
(image-mode-map): New menu item "Show as Hex".
(image-mode): Update doc string and echo-area message.
(image-toggle-display): Support toggle to hex.  (Bug#22453)

* doc/emacs/files.texi (File Conveniences): Document
'image-toggle-hex-display'.