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

[JayDi85]

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

Current game under control support:

• human over human -- fully supported;
• 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.