* lisp/image-mode.el: Add prefix key 's' and reduce dependency on ImageMagick.

* lisp/image-mode.el: Add prefix key 's' and reduce dependency on ImageMagick.

* lisp/image-mode.el (image-mode-map): Regroup existing keybindings and
add new ones with the prefix key 's'.
Remove condition ":visible (eq image-type 'imagemagick)" from menu.
(image-toggle-display-image): Don't rotate again after user rotated manually.
(image-transform-check-size): Remove check for imagemagick.
(image-transform-properties, image-transform-set-scale)
(image-transform-fit-to-height, image-transform-fit-to-width)
(image-transform-set-rotation, image-transform-reset):
Remove mentions of ImageMagick from docstrings since these commands
now work without ImageMagick.