Populate the window title whenever worktrees or active path change

* Refactor the way the project's active entry is assigned. Assign it
  together with the window title, as opposed to on every notification
  from a pane.
* Emit the ActiveItem event from panes consistently, even when adding
  the first item to an empty pane.

crates/zed/src/menus.rs

@@ -229,6 +229,10 @@ pub fn menus() -> Vec<Menu<'static>> {
                 },
             ],
         },
+        Menu {
+            name: "Window",
+            items: vec![MenuItem::Separator],
+        },
         Menu {
             name: "Help",
             items: vec![MenuItem::Action {