[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)
function set_tab(tabname, cookiename) {
// set appropriate cookie
loadRemote('json', ajaxFuncArgs('set_cookie', null, cookiename, tabname));
// trigger show + tabname event
trigger_load(tabname);
}