Add UI mechanism to take manual control over a player

[Debilski]

The idea would be that we stop the game in UI and then have the option to take over the blue or the red team. This would terminate the process for that player (unless we find a sensible way to keep it running with the manually entered moves) but the other player would still be reacting to the moves that we enter.

I assume that this could be helpful for certain debugging situations and behavioural tests without having to prepare the situation in code.