Generalize "animated" images to "multi-frame" images

Generalize "animated" images to "multi-frame" images

* lisp/image.el (image-animated-types): Remove.
(image-multi-frame-p): Rename from image-animated-p, and generalize.
(image-animated-p): Make obsolete alias.
(image-animate, image-nth-frame, image-animate-timeout):
Use image-multi-frame-p.
(image-animate-timeout): If no delay, use image-default-frame-delay.

* lisp/image-mode.el (image-mode, image-toggle-animation):
Use image-multi-frame-p.
(image-mode): Adjust startup message for a multi-frame image.

* lisp/gnus/shr.el (shr-put-image): Only animate images that specify a delay.
This is consistent with the old image-animated-p behavior.

* etc/NEWS: Add placeholder for this.