Introduce a new game backend function (there seem to have been a lot

of these recently) whose job is to update a game_ui to be consistent
with a new game_state. This is called by midend.c in every situation
where the current game_state changes _other_ than as a result of
make_move (Undo, Redo, Restart, Solve).

The introduction of this function allows a game_ui to contain
information about selections or highlights within a game_state which
simply wouldn't make sense when transferred to another game_state.
In particular, I've used it to fix a subtle bug in Solo whereby,
although you couldn't right-click to pencil-mode highlight a filled
square, you could _get_ a pencil-mode highlight in a filled square
if you used Undo and Redo. (Undo to before the square was filled,
right-click to highlight it, then Redo. Alternatively, left-click
and clear the square, right-click to highlight it, then Undo.)

[originally from svn r5912]

puzzles.h

@@ -252,6 +252,8 @@ struct game {
     char *(*text_format)(game_state *state);
     game_ui *(*new_ui)(game_state *state);
     void (*free_ui)(game_ui *ui);
+    void (*changed_state)(game_ui *ui, game_state *oldstate,
+                          game_state *newstate);
     game_state *(*make_move)(game_state *from, game_ui *ui, game_drawstate *ds,
                              int x, int y, int button);
     void (*size)(game_params *params, int *x, int *y);