Added explicit DEFGENERIC form for Y-INVERTING-TRANSFORMATION-P.

The generic function Y-INVERTING-TRANSFORMATION-P is McCLIM specific,
so it does not have a DEFGENERIC form in decls.lisp.  By having one
here, we avoid a style warning from some Common Lisp compilers.

Also, included a comment explaining why that DEFGENERIC form is
included here rather than in decls.lisp.
Updated copyright and email address.
Removed unused log and improved comments.