Sets the CSS class that provides a background image to use as the button's icon. This method also changes the
value of the {@link #iconCls} config internally.
@param {String} cls The CSS class providing the icon image
@return {Ext.button.Button} this
function(cls) {
        cls = cls || '';
        var me = this,
            btnIconEl = me.btnIconEl,
            oldCls = me.iconCls || '';
            
        me.iconCls = cls;
        if (oldCls != cls) {
            if (btnIconEl) {
                // Remove the previous iconCls from the button
                btnIconEl.removeCls(oldCls);
                btnIconEl.addCls(cls || '');
                me.setComponentCls();
                if (me.didIconStateChange(oldCls, cls)) {
                    me.updateLayout();
                }
            }
            me.fireEvent('iconchange', me, oldCls, cls);
        }
        return me;
    }