Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

shr.el: Start implementation.
shr.el: Continue implementation.
gnus-gravatar.el (gnus-gravatar-insert): Adjust character where we should go backward.
shr.el: Minimally useful state achieved.
mm-decode.el (mm-text-html-renderer): Switch to using shr.el for HTML rendering.
shr.el: (shr-insert): Add a newline after every picture before text.
gnus.texi (Splitting Mail): Really fix the @ref syntax.
shr.el (shr-add-font): Use overlays for combining faces.
shr.el (shr-add-font): Use overlays for combining faces.
shr.el (shr-insert): Pass upwards the text start point.
gnus-util.el: Reintroduce multiple completion functions.