Robustify handling of activation & focus on Windows

[RandomShaper]

I found the need for this while assessing #80548. The central ideas are these:

• In Godot we don't really need to tell activation apart from focus. It's the same. The only need, given how the handle child windows, is to explicitly set focus to any window just activated (see the linked PR).
• The handling of application-wide focus is reimplemented here in a more reliable way.
• Certain timer hack is removed (while the issue it intended to fix, Popup menus do not close when other is opened (DisplayServer regression) #44953, is still gone).

[akien-mga]

CC @bruvzg

[bruvzg]

Seems to be working fine, I have not found any issues. #44953 should not happen, since popups no longer take focus.

[akien-mga]

Needs rebase.

[RandomShaper]

Rebased. I will retest it again. Some additional re-testing from others would be appreciated.

[akien-mga]

Thanks!