Reset Memory::popup to None if a popup was abandoned

[juancampa]

• Closes any_popup_open() remains true even when there are no popups #3657

BEFORE

Screenshot.2024-06-23.at.18.41.41.mp4

AFTER

Screenshot.2024-06-23.at.18.56.01.mp4

Notice that WRONG flashes for one frame. That's expected because on the frame the combo box disappears, any_popup_open still returns true, since we don't know it has disappeared until the end of the frame.

[juancampa]

I'm using an AtomicBool here because I didn't want to change is_popup_open to take &mut self.

An alternative is to add another function, something like retain_popup(&mut self) that explicitly sets a retained bool to true. But would will be a breaking change and existing apps will need to call it or popups will only last for one frame.

I'm open to suggestions.