Sets the CSS class that provides the icon image for this header.  This method will replace any existing
icon class if one has already been set.
@param {String} cls The new CSS class name
function(cls) {
        var me = this,
            isEmpty = !cls || !cls.length,
            iconCmp = me.iconCmp;

        me.iconCls = cls;
        if (!me.iconCmp && !isEmpty) {
            me.initIconCmp();
            me.insert(0, me.iconCmp);
        } else if (iconCmp) {
            if (isEmpty) {
                me.iconCmp.destroy();
                delete me.iconCmp;
            } else {
                iconCmp.removeCls(iconCmp.iconCls);
                iconCmp.addCls(cls);
                iconCmp.iconCls = cls;
            }
        }
    }