Mark popup as open (toggle toward open state). Popups are closed when user click outside, or activate a pressable item, or CloseCurrentPopup() is called within a BeginPopup()/EndPopup() block. Popup identifiers are relative to the current ID-stack (so OpenPopup and BeginPopup needs to be at the same level). One open popup per level of the popup hierarchy (NB: when assigning we reset the Window mem