addition to the `game_state'. The new structure is intended to
contain ephemeral data pertaining to the game's user interface
rather than the actual game: things stored in the UI structure are
not restored in an Undo, for example.
make_move() is passed the UI to modify as it wishes; it is now
allowed to return the _same_ game_state it was passed, to indicate
that although no move has been made there has been a UI operation
requiring a redraw.
[originally from svn r4207]
