[new feature] DropdownItem: add open & close event

packages/dropdown-menu/en-US.md

@@ -98,6 +98,9 @@ export default {
 | Event | Description | Arguments |
 |------|------|------|
 | change | Triggered select option and value changed | value |
+| open | Triggered when open menu | - |
+| opened | Triggered when opened menu | - |
+| close | Triggered when close menu | - |
 
 ### DropdownItem Methods
 

packages/dropdown-menu/zh-CN.md

@@ -100,6 +100,9 @@ export default {
 | 事件名 | 说明 | 回调参数 |
 |------|------|------|
 | change | 点击选项导致 value 变化时触发 | value |
+| open | 打开菜单栏时触发 | - |
+| opened | 打开菜单栏且动画结束后触发 | - |
+| close | 关闭菜单栏时触发 | - |
 
 ### DropdownItem 方法