AI: computer player must be able to take control over another player (human or computer)

It's about "game under control" effects. Activate as another player or other controller/opponent choices works fine.

Current game under control support:
* [x] human over human -- fully supported;
* [x] human over computer -- fully supported;
* [ ] computer over computer -- unsupported and disabled;
* [ ] computer over human -- unsupported and disabled;

See problems and comments in #12878

Current code must be stable for both computer and human games (no more game freezes), so it's a low priority problem now. Computer support must be implemented same way as "human over computer" -- as special proxy player object but with additional changes in AI logic to "revert" good/bad choices and game score calcs.

Start with the problems and comments in issue #12878, then trace the existing human-over-computer game-under-control support described here. The work is done when computer-over-computer and computer-over-human control are enabled without freezes, with AI choices and game score calculations correctly reverted as needed.

Written by the indexing model from the issue text.