Select: fix clear value & remote label

select improve

DatePicker: disabled year or month, fixed #758

Message/Notification: manage z-index with PopupManager

Table/MessageBox/Tree: remove object-assign

Doc: add anchor

Table: add current-change event & highlight-current-row prop.

* Fix upload docs

* fix upload docs

fix upload docs

dropdown-item add disabled && divided

fix build theme script

Table: support header mousewheel scroll.

Table: clear selection after change data, fixed #813.

Loading: fix a potential node removal bug

Table: let table compatible with vuex.

Message, Notification: fix transitionend bug

问题描述：
如果我在组件中加入不同的transition或者transition: all
duration的形式，那么this.$el.addEventListener(‘transitioned’,
handeler)就会监听到多次事件，从而导致多次删除，因为节点已经从dom中删除，
所以最后报错：
’main.vue?8f52:44
Uncaught TypeError: Cannot read property 'removeChild' of null’。
解决办法：
就是在第一次监听到动画结束事件时，将监听器移除即可。

doc: add custom theme