[新增功能](master): 全局添加回到顶部功能

Author: elunez

src/layout/components/AppMain.vue

@@ -5,6 +5,7 @@
         <router-view :key="key" />
       </keep-alive>
     </transition>
+    <el-backtop :bottom="50" :right="40"><i class="el-icon-caret-top" /></el-backtop>
     <div v-if="$store.state.settings.showFooter" id="el-main-footer">
       <span v-html="$store.state.settings.footerTxt" />
       <span> ⋅ </span>