[FEATURE] Added v:page.header.canonical to render canonical tag

Hey,

the attached commit adds the possibility to include a canonical link tag. This will magically (not really) render it as a tag on the spot or via the page renderer. The link is chosen based on the language settings for the page as defined by ``TMENU``. I included that part in the ``PageSelectService`` since in ``vhs`` the very same thing is scattered around classes.