Supports stacked message boxes and removes the according workaround.
Change structree / civinfo switch-dialog code from rP21339 to perform the callback for page that actually registered the callback.
Ensure the parent that the callbackhandler is always called if the page is closed.
Merge PopGuiPage and PopGuiPageCB following that choice, incidentally leaving cleaner code.
Differential Revision: https://code.wildfiregames.com/D1684
Comments by: Yves, Vladislav, wraitii, leper