zed: Persist window stack order across restarts (#15419)

This changes the workspace/session serialization to also persist the
order of windows across restarts.

Release Notes:

- Improved restoring of windows across restarts: the order of the
windows is now also restored. That means windows that were in the
foreground when Zed was quit will be in the foreground after restart.
(Right now only supported on Linux/X11, not on Linux/Wayland.)

Demo:



https://github.com/user-attachments/assets/0b8162f8-f06d-43df-88d3-c45d8460fb68

crates/gpui/src/platform/linux/x11/client.rs

@@ -1345,6 +1345,48 @@ impl LinuxClient for X11Client {
                 .map(|window| window.handle())
         })
     }
+
+    fn window_stack(&self) -> Option<Vec<AnyWindowHandle>> {
+        let state = self.0.borrow();
+        let root = state.xcb_connection.setup().roots[state.x_root_index].root;
+
+        let reply = state
+            .xcb_connection
+            .get_property(
+                false,
+                root,
+                state.atoms._NET_CLIENT_LIST_STACKING,
+                xproto::AtomEnum::WINDOW,
+                0,
+                u32::MAX,
+            )
+            .ok()?
+            .reply()
+            .ok()?;
+
+        let window_ids = reply
+            .value
+            .chunks_exact(4)
+            .map(|chunk| u32::from_ne_bytes(chunk.try_into().unwrap()))
+            .collect::<Vec<xproto::Window>>();
+
+        let mut handles = Vec::new();
+
+        // We need to reverse, since _NET_CLIENT_LIST_STACKING has
+        // a back-to-front order.
+        // See: https://specifications.freedesktop.org/wm-spec/1.3/ar01s03.html
+        for window_ref in window_ids
+            .iter()
+            .rev()
+            .filter_map(|&win| state.windows.get(&win))
+        {
+            if !window_ref.window.state.borrow().destroyed {
+                handles.push(window_ref.handle());
+            }
+        }
+
+        Some(handles)
+    }
 }
 
 // Adatpted from: