perf[el-dragDialog]: add dragDialog callback function

src/directive/el-dragDialog/drag.js

@@ -1,5 +1,5 @@
 export default{
-  bind(el, binding) {
+  bind(el, binding, vnode) {
     const dialogHeaderEl = el.querySelector('.el-dialog__header')
     const dragDom = el.querySelector('.el-dialog')
     dialogHeaderEl.style.cssText += ';cursor:move;'
@@ -63,6 +63,9 @@ export default{
 
         // 移动当前元素
         dragDom.style.cssText += `;left:${left + styL}px;top:${top + styT}px;`
+
+        // emit onDrag event
+        vnode.child.$emit('dragDialog')
       }
 
       document.onmouseup = function(e) {