Introduce an environment variable setting (SLANT_SWAP_BUTTONS=yes)

to reverse the effect of the mouse buttons. Gareth has been
complaining about this for days: apparently he finds precisely the
opposite control system intuitive to me.

This is a horrendous hack, and pushes me one step closer to losing
my temper and designing a proper preferences architecture.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@6168 cda61777-01e9-0310-a592-d414129be87e