Redesign recent documents role API

- For more flexibility in designing the recent documents menu, now there can be one menu item (known issue: you may specify more but AppKit doens't display more than one) in the submenu list, rather than separating the labels with a tab.
- Also added is the role `clearrecent` that can be given to a menu item that can clear the recent documents.

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

@@ -103,7 +103,10 @@ const roles = {
     label: 'Services'
   },
   openrecent: {
-    label: 'Open Recent\tClear Menu'
+    label: 'Open Recent'
+  },
+  clearrecent: {
+    label: 'Clear Menu'
   },
   startspeaking: {
     label: 'Start Speaking'