From PR godotengine#38727 which was reverted in godotengine#41373 because of regressions in Ubuntu
with Gnome.

Co-authored-by: Lorenzo Cerqua <lorenzocerqua@tutanota.com>

Previously, only the direct parent were taken into account.

Popups like contextual menus could stay open if an ancestor which is
not a direct parent was focused.

Reproduction steps (any platform):
- Select a node in the scene tree
- Left click the node to start renaming
- Right click to open the copy/paste contextual menu
- Left click in the scene tree to deselect the node

Also closing popup when focusing out of the application, without waiting
for the parent to get focus to do so.

Now using override_redirect for menu & tooltip popups to prevent the WM from
interfering with them, so we have more control over focus management
and avoid a delay before they show up.

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.