Introduce openrecent role

- Does not yet accept any options except label--"Clear Menu" can't be modified through exposed node API
- If `openrecent` is present across multiple menu items, the last seen will be visible in the main menu due to that it is designed to behave like a singleton

lib/browser/api/menu-item-roles.js

@@ -102,6 +102,9 @@ const roles = {
   services: {
     label: 'Services'
   },
+  openrecent: {
+    label: 'Open Recent'
+  },
   startspeaking: {
     label: 'Start Speaking'
   },