Framework alteration: we now support a `game_ui' structure in

Framework alteration: we now support a `game_ui' structure in
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.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@4207 cda61777-01e9-0310-a592-d414129be87e