Support opening action items in a new window

Description

The new navigation broke PopupMenuItem functionality, as the items no longer opened in a new window.

In general, window.open should be avoided as it is bad for accessibility and can have performance implications. Instead, you should set target="_blank" for action items in the new navigation to support opening in a new window.

For this reason, PopupMenuItem should be obsoleted, and the use of UrlMenuItem with the Target property set should be promoted.