Rework how Gnus is supposed to be able to display all the images in HTML.

Rework how Gnus is supposed to be able to display all the images in HTML.

shr.el (shr-tag-img): Put a displayer in the text property.
gnus-util.el (gnus-find-text-property-region): New utility function.
gnus-html.el (gnus-html-display-image): Make the alt optional.
gnus-html.el (gnus-html-show-images): Remove.
gnus-art.el (gnus-article-show-images): New, more general function.
gnus-html.el, shr.el: Use image-url instead of gnus-image-url to unify the image url text properties.