Navigation: Implement drag and drop for reordering navigation block links

[jeryj]

There currently is not way to move a top-level item into a submenu item, or reposition a submenu item and its children outside of a submenu.

Navigation link blocks should be able to be moved around via a drag handle and placed in the top-level or nested within a submenu.

Describe the solution you'd like
Show the drag handle between the arrows (as of this writing, it's hidden via CSS) and allow items to be drag/dropped anywhere in the navigation block.

Describe alternatives you've considered
For now, we've hidden this functionality, as you can move items within their current level using the arrows.

[tellthemachines]

It's already possible to move Navigation links between nesting levels with drag and drop; can we close this issue now, or is there anything left to address?

[draganescu]

I think we can close this issue.