Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

shr.el: Rename the tag functions a bit, and add some new ones.
gnus-sum.el (gnus-summary-select-article-buffer): If the article buffer isn't shown, then select the current article first instead of bugging out.
gnus-sum.el (gnus-summary-select-article-buffer): Show both the article and summary buffers again.
shr.el (shr-tag-blockquote): Convert name.
shr.el (shr-rescale-image): Use the right image-size variant.
shr.el (shr-tag-p): Don't insert newlines at the start of the buffer.
shr.el: Implement indentation in blockquotes.
gnus-sum.el (gnus-summary-select-article-buffer): Really select the article buffer again.
shr.el (shr-ensure-paragraph): Don't insert newlines on empty tags at the beginning of the buffer.
gnus-ems.el, gnus-util.el, mm-decode.el, mm-view.el: Add resize for large images in mm.
gnus-html.el (gnus-html-put-image): Use gnus-rescale-image.
shr.el (shr-tag-p): Don't insert newlines on empty tags at the beginning of the buffer.
gnus-ems.el, gnus-html.el, gnus-util.el, mm-decode.el, mm-view.el: Support image resizing.
shr.el: Add headings.
shr.el (shr-ensure-paragraph): Actually work.
shr.el (shr-tag-li): Make <ul> prettier.
shr.el (shr-insert): Get white space at the beginning/end of elements right.
shr.el (shr-tag-li): Tweak <li> rendering.
shr.el (shr-tag-p): Collapse subsequent <p>s.
shr.el (shr-ensure-paragraph): Don't insert double line feeds after blank lines.
shr.el (shr-tag-h6): Add.
shr.el (shr-insert): \t is also space.