[UX] Allow site admins to specify which menu is to be used in the admin bar.

This is basically a request to add https://www.drupal.org/project/admin_menu_source into Backdrop core:

This would be a nice solution for what this person is trying to achieve in https://forum.backdropcms.org/forum/flyout-menu (aside from the vertical placement of the admin bar).

This is one of the things I'm planning to discuss during the Content Author-centric UX improvements in core session in the upcoming Backdrop Live event.