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