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.)

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5888 cda61777-01e9-0310-a592-d414129be87e