dwz navtab 限制打开数量实例

  openTab: function(tabid, url, options){ //if found tabid replace tab, else create a new tab.
    var op = $.extend({title:"New Tab", data:{}, fresh:true, external:false}, options);
    //这里判定如果未加载该 navtab 并且超出最大允许的navtab数量
    if( navTab._getTabs().length > 4 )
    {
      //如果未加载过,或者已在main里面显示的
      if( navTab._indexTabId(tabid) < 0 )
      {
        navTab._getTabs().eq(0).attr('true_tabid',tabid);
        tabid = 'main';
      }
    }
    else
    {
      var real_tabid = navTab._getTabs().eq(0).attr('true_tabid');
      if( real_tabid && real_tabid == tabid )
      {
        tabid = 'main';
      }
    }
    var iOpenIndex = this._indexTabId(tabid);

    if (iOpenIndex >= 0){
      var $tab = this._getTabs().eq(iOpenIndex);
      var span$ = $tab.attr("tabid") == this._op.mainTabId ? "> span > span" : "> span";
      $tab.find(">a").attr("title", op.title).find(span$).text(op.title);
      var $panel = this._getPanels().eq(iOpenIndex);
      if(op.fresh || $tab.attr("url") != url) {
        $tab.attr("url", url);
        if (op.external || url.isExternalUrl()) {
          $tab.addClass("external");
          navTab.openExternal(url, $panel);
        } else {
          $tab.removeClass("external");
          $panel.ajaxUrl({
            type:"GET", url:url, data:op.data, callback:function(){
              navTab._loadUrlCallback($panel);
            }
          });
        }
      }
      this._currentIndex = iOpenIndex;
    } else {
      var tabFrag = '