feat: refine theme api (#1319)

Author: ulivz

CHANGELOG.md

@@ -22,51 +22,6 @@
 ### Features
 
 * **$core:** support global layout (close: [#1226](https://github.com/vuejs/vuepress/issues/1226)) ([c91f55a](https://github.com/vuejs/vuepress/commit/c91f55a))
-
-  From now on, users have the ability to use a custom global layout component via [siteConfig](https://v1.vuepress.vuejs.org/miscellaneous/glossary.html#siteconfig) or [themeEntryFile](https://v1.vuepress.vuejs.org/miscellaneous/glossary.html#themeentryfile):
-  
-  ```js
-  module.exports = {
-    globalLayout: '/path/to/your/global/vue/sfc'
-  }
-  ```
-
-  Here is the [content of default global layout component](https://github.com/vuejs/vuepress/blob/master/packages/%40vuepress/core/lib/app/components/GlobalLayout.vue), an example of setting global header and footer:
-  
-  ```vue
-  <template>
-    <div id="global-layout">
-      <header><h1>Header</h1></header>
-      <component :is="layout"/>
-      <footer><h1>Footer</h1></footer>
-    </div>
-  </template>
-  
-  <script>
-  export default {
-    computed: {
-      layout () {
-        if (this.$page.path) {
-          if (this.$vuepress.isLayoutExists(this.$page.frontmatter.layout)) {
-            return this.$page.frontmatter.layout
-          }
-          return 'Layout'
-        }
-        return 'NotFound'
-      }
-    }
-  }
-  </script>
-  ```
-  
-  Also, you can follow the convention, directly create a component `.vuepress/components/GlobalLayout.vue` or `themePath/layouts/GlobalLayout.vue` without any config. the loading priority is as follows: 
-  
-  - siteConfig
-  - siteAgreement
-  - themeEntryFile
-  - themeAgreement
-  - default
-
 * **$theme-default:** disable search box via frontmatter (close: [#1287](https://github.com/vuejs/vuepress/issues/1287)) ([#1288](https://github.com/vuejs/vuepress/issues/1288)) ([54e9eb0](https://github.com/vuejs/vuepress/commit/54e9eb0))
 
 

__mocks__/vuepress-theme-child/Layout.vue

@@ -0,0 +1,3 @@
+module.exports = {
+  extend: 'vuepress-theme-parent'
+}

__mocks__/vuepress-theme-child/components/Home.vue

@@ -0,0 +1 @@
+module.exports = {}