On Windows, WINDOW_EVENT_FOCUS_IN was never sent by the display server
for popups, because WM_ACTIVATE events are received during the call to
_update_window_style, which happened before the callbacks were set.
This was causing some issues with the way Popup is now handling closing on
parent focus.
Now _update_window_style is only called during show_window, after Window
initialized callbacks.
bb306750ce