MessageBox: add 'message' support for VNode

[Leopoldthecoder]

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用了 Notification 的方法

[leo4stone]

这种支持方式没解决根本问题啊
用messagebox就是为了全局使用，有时候缺少vue对象，根本无法使用vue.createElement去创建vnode，而导致无法实现支持html的string内容

[Leopoldthecoder]

@wfesorg

import Vue from 'vue'
const Ctor = Vue.extend()
const createElement = new Ctor().$createElement

Now you have createElement