[bug report] dropdown cannot be displayed in full screen. There is no appendtobody property like select

### Element UI version
2.15.3

### OS/Browsers version
win10/Microsoft Edge v91.0.864.64

### Vue version
2.6.12

### Reproduction Link
https://codepen.io/dodu2014/pen/yLbOMBK

### Steps to reproduce
点击全屏按钮,进入全屏
select 下拉菜单正常
dropdown 下拉菜单不能正常显示, 原因是 dropdown-menu 绘制到#app元素以外了
能否让 dropdown 也支持 popper-append-to-body

### What is Expected?
期待 dropdown 支持 popper-append-to-body 属性配置, 和 select 的 popper-append-to-body 这个属性效果一致

### What is actually happening?
dropdown 不支持 popper-append-to-body