Better mouse button handling in Mines:

- middle button now also triggers the clear-around-square action
 - a special-case handler in midend_process_key() arranges that the
   left button always trumps the right button if both are pressed
   together, meaning that Windows Minesweeper players used to
   pressing L+R to clear around a square should still be able to do
   so without any strange behaviour.
(The latter touches all game backends, yet again, to add a field to
the game structure which is zero in everything except Mines.)

[originally from svn r5888]

nullgame.c

@@ -254,4 +254,5 @@ const struct game thegame = {
     game_flash_length,
     game_wants_statusbar,
     FALSE, game_timing_state,
+    0,				       /* mouse_priorities */
 };