Makes the given component active (makes it the visible card in the GroupTabPanel's CardLayout)
@param {Ext.Component} cmp The component to make active
function(cmp) {
        var me = this,
            newTab = cmp,
            oldTab;

        if(Ext.isString(cmp)) {
            newTab = Ext.getCmp(newTab);
        }

        if (newTab === me.activeTab) {
            return false;
        }

        oldTab = me.activeTab;
        if (me.fireEvent('beforetabchange', me, newTab, oldTab) !== false) {
             me.activeTab = newTab;
             if (me.rendered) {
                 me.down('container[baseCls=' + Ext.baseCSSPrefix + 'grouptabcontainer' + ']').getLayout().setActiveItem(newTab);
             }
             me.fireEvent('tabchange', me, newTab, oldTab);
         }
         return true;
    }