document.getElementById("column-left").getElementsByTagName("header")[0].onclick = function(){
    if(document.getElementById('vselect') === null){
         var select = document.createElement("select");
         select.setAttribute('id','vselect');
         select.options[0] = new Option("加载项1", "");
         select.options[1] = new Option("加载项2", "");
         this.appendChild(select);
    }

}