fix order of exit/close window systems

[mockersf]

Objective

Fixes #5384 and maybe other issues around window closing/app not exiting

Solution

There are three systems involved in exiting when closing a window:

• close_when_requested asking Winit to close the window in stage Update
• exit_on_all_closed exiting when no window remains opened in stage Update
• change_window removing windows that are closed in stage PostUpdate

This ordering meant that when closing a window, we had to run one more frame to actually exit. As there was no window, panics could occur in user code assuming there was a window. In case of Bevy app using a low power options, that means waiting for the timeout before actually exiting the app (60 seconds by default)

This PR changes the ordering so that exit_on_all_closed happens after change_window in the same frame, so there isn't an extra frame without window