ComboBox没的click事件，下拉按钮的点击事件处理是通过重载从Ext.form.TriggerFiled中继承的方法onTriggerClick实现，但是这种实现破坏了combobox原有的功能。
var combo = new Ext.form.ComboBox({
store: store,
displayField:'state',
typeAhead: true,
mode: 'local',
forceSelection: true,
triggerAction: 'all',
emptyText:'Select a state...',
selectOnFocus:true,
applyTo: 'local-states',
onTriggerClick:function(){
alert('abc');
}
});
