Add functions to change the speed of animated images

Add functions to change the speed of animated images

* lisp/image-mode.el (image-mode-map): Add menu items to reverse,
increase, decrease, reset animation speed.
(image--set-speed, image-increase-speed, image-decrease-speed)
(image-reverse-speed, image-reset-speed): New functions.
(image-mode-map): Add bindings for speed commands.

* lisp/image.el (image-animate-get-speed, image-animate-set-speed):
New functions.
(image-animate-timeout): Respect image :speed property.

* etc/NEWS: Mention this.