[ajax] refactor/cleanup low-level ajax functions * loadxhtml/replacePageChunck/reload_component/reload_box deprecated in favor of loadXHTML / ajaxFuncArgs * some other cleanups in cubicweb.ajax.js * add js_render which will replace js_component (more generic, nicer argument passing handling)

<div id="ajaxroot">
  <h1>Hello</h1>
</div>