Allow setting the charset in eww

Fixes: debbugs:16225

* net/eww.el (eww-set-character-encoding): New command and keystroke.
(eww-display-raw): Use it.
(eww-mode-map): Bind backtab to shr-previous-link

* net/eww.el (eww-mode-map): Bind backtab to shr-previous-link
* net/eww.el (eww-add-bookmark): Fix bookmark titles.
Document the eww Desktop stuff

Fixes: debbugs:18010

* eww.texi (Advanced): Mention the Desktop stuff (bug#18010).
Make `M-x eww' default to the URL under point

* net/eww.el (eww-suggest-uris): New variable.
(eww-suggested-uris): New function.
(eww): Default to URL under point.
(eww-links-at-point): New function.
* net/eww.el (eww-display-html): Decode the document-defined charset.