Move image commands to the 'i' sub map

Move image commands to the 'i' sub map

* doc/lispref/display.texi (Showing Images): Adjust.
* lisp/image.el (image-map): Move all keys under the "i" prefix.
(image--repeat-map): New map.
(image-increase-size, image-rotate, image-decrease-size): Make
repeatable.
(image--delayed-change-size): New function.