[plt-translators] String constants GC

I've walked over the whole PLT tree and looked for string constants
that are actually used.  I found more than a hundred that are not
used, so it's probably a good idea to do some cleaning.  I don't know
the policy that each translation uses, so instead of removing the
entries, I commented them all with a ";!" prefix.  I did the same on
the original enlish file.

Feel free to remove whatever if you prefer to.

(BTW, I also renamed `wizard-back' to `dialog-back', since it is used
(once) in a generic non-wizard way.)

-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                  http://www.barzilay.org/                 Maze is Life!