How to customize Toolbar of admin screen.
Can be add items
You can be able to setting toolbar items and custom HTML, custom link, Group.
Exist menu items is can change the menu title and icon.
Exist menu item
You can be able to custom link and external link for setting toolbar items. And use the shortcodes.
You can be able to custom HTML. And use the shortcodes.
Custom Menu UI
This has no effect at now, it is for future addition.
This customize toolbar of admin is use the cache. If does not change the menu items, please re-Save and check the cache time.