[ticket/12369] Add template var BODY_CLASS to HTML body

For extensions, it might be good to be able to add CSS classes to the `<body>` element, without having to resort to JS. Modifying the `{SCRIPT_NAME}` or `{S_CONTENT_DIRECTION}` via `$this->template->append_var` is bound to result in problems.

I was thinking: either we would include a template event in the `<body class="event-here">`, or we simply use a new template var, which can be modified easily.

With the inclusion of the new `core.page_header_after` event, ext authors can simply use: `$this->template->append_var("BODY_CLASS", ' my-body-class');`

~~But I'm not sure whether or not to also add this template var to the ACP header output.~~

http://tracker.phpbb.com/browse/PHPBB3-12369

PHPBB3-12369