Introduced a new function in every game which formats a game_state

as text. This is used by front ends to implement copy-to-clipboard.
Currently the function does nothing (and is disabled) in every game
except Solo, but it's a start.

[originally from svn r5724]

pattern.c

@@ -648,6 +648,11 @@ static void free_game(game_state *state)
     sfree(state);
 }
 
+static char *game_text_format(game_state *state)
+{
+    return NULL;
+}
+
 struct game_ui {
     int dragging;
     int drag_start_x;
@@ -1029,6 +1034,7 @@ const struct game thegame = {
     new_game,
     dup_game,
     free_game,
+    FALSE, game_text_format,
     new_ui,
     free_ui,
     make_move,