Only the player who did the latest action should be allowed to undo it

[michaeljb]

Discourage players from thinking they should be able to just undo turns however they want

Complicating considerations:

• undo is still possible via master mode; players who want this feature would want master mode undo actions to be logged
• when dealing with undo actions in this way, would similarly need to consider redo actions
• possible end turn button? this would be a very large change, would flow better if it was configurable in user settings (i.e., allow users to keep current behavior if they wished); if disabled, the end turn action would need to be automatically added to the actions list, not simply processed; also would not want end turn blocking if no actions were possible during the turn, only if actions were actually taken