同步至Element UI 2.4.8版本

Merge branch 'master' into current

* master: (56 commits)
  [release] 2.4.8
  [build] 2.4.8
  changelog for 2.4.8 (ElemeFE#12950)
  Tabs: scroll to active tab (ElemeFE#12948)
  Tree: fix type definition (ElemeFE#12941)
  Table: fix table column default alignment (ElemeFE#12578)
  Table: called render header in index column (ElemeFE#12914)
  Input: s autocomplete on textarea (ElemeFE#12917)
  DatePicker: validate input date  (ElemeFE#12898)
  chore: update date-picker comment notes (ElemeFE#12899)
  test: update test case for form (ElemeFE#12886)
  ButtonGroup: fix Dropdown style in ButtonGroup(ElemeFE#12815) (ElemeFE#12819)
  Form: emits error message on validate event (ElemeFE#12860)
  doc: update i18n doc for Serbian language (ElemeFE#12867)
  Tabs: fix order of TabNav when array is sorted (ElemeFE#12846)
  i18n: add Serbian translation (ElemeFE#12863)
  Tree: show indicator in drop node (ElemeFE#12854)
  Upload: add missing definition (ElemeFE#12839)
  i18n: update Greek translation (ElemeFE#12830)
  Dialog: add opened event (ElemeFE#12828)
  ...

# Conflicts:
#	CHANGELOG.zh-CN.md
#	README.md
#	examples/docs/zh-CN/form.md
#	examples/versions.json
#	package.json
#	packages/date-picker/src/basic/date-table.vue
#	packages/date-picker/src/basic/month-table.vue
#	packages/date-picker/src/panel/date-range.vue
#	packages/form/src/form-item.vue
#	packages/theme-chalk/package.json
#	packages/theme-chalk/src/button.scss
#	src/index.js

examples/versions.json

@@ -1 +1 @@
-{"1.4.25":"1.4"}
+{"1.4.26":"1.4"}

src/index.js

@@ -194,7 +194,7 @@ if (typeof window !== 'undefined' && window.Vue) {
 }
 
 module.exports = {
-  version: '1.4.25',
+  version: '1.4.26',
   locale: locale.use,
   i18n: locale.i18n,
   install,