Add canonical link to XWiki pages to reduce duplicate content indexation by search engines

Pretty often when googling something related XWiki, results contain XWiki pages entries with ?viewer=code or ?viewer=XXX parameters.

To avoid this we should specify the canonical URL in view mode

See http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html

(Yahoo and Bing understand this directive as well)