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.

[originally from svn r4207]

sixteen.c

@@ -358,7 +358,16 @@ void free_game(game_state *state)
     sfree(state);
 }
 
-game_state *make_move(game_state *from, int x, int y, int button)
+game_ui *new_ui(game_state *state)
+{
+    return NULL;
+}
+
+void free_ui(game_ui *ui)
+{
+}
+
+game_state *make_move(game_state *from, game_ui *ui, int x, int y, int button)
 {
     int cx, cy;
     int dx, dy, tx, ty, n;
@@ -534,7 +543,8 @@ static void draw_arrow(frontend *fe, int x, int y, int xdx, int xdy)
 }
 
 void game_redraw(frontend *fe, game_drawstate *ds, game_state *oldstate,
-                 game_state *state, float animtime, float flashtime)
+                 game_state *state, game_ui *ui,
+                 float animtime, float flashtime)
 {
     int i, bgcolour;