refactor xhtml TOC creation into a class function, FS#1058

- changed inc/parser/xhtml.php, render_TOC() function
  it now takes a toc array as a parameter
- refactor render_TOC()  and _tocitem() into static class functions

The xhtml renderer can build its TOC using $this->render_TOC($this->toc)
Templates can create a separate TOC using
  echo Doku_Renderer_xhtml::render_TOC(p_get_metadata($ID, 'description
tableofcontents'));

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