Enable and disable nav buttons based on pane's navigation stack

Also, make the `NavHistory` type private to the `workspace` crate.
Expose only the `ItemNavHistory` type, via a method on Pane called
`nav_history_for_item`.
a378ec49ecMax Brunsfeld committed on 7/6/2022, 10:46:43 PMparent7e5cf66
6 files changedLine totals unavailable