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/app.rs

@@ -469,6 +469,15 @@ impl AppContext {
             .collect()
     }
 
+    /// Returns the window handles ordered by their appearance on screen, front to back.
+    ///
+    /// The first window in the returned list is the active/topmost window of the application.
+    ///
+    /// This method returns None if the platform doesn't implement the method yet.
+    pub fn window_stack(&self) -> Option<Vec<AnyWindowHandle>> {
+        self.platform.window_stack()
+    }
+
     /// Returns a handle to the window that is currently focused at the platform level, if one exists.
     pub fn active_window(&self) -> Option<AnyWindowHandle> {
         self.platform.active_window()