Make properties discoverable from Python:

Make properties discoverable from Python:

- property() now takes 4 keyword arguments:  fget, fset, fdel, doc.
  Note that the real purpose of the 'f' prefix is to make fdel fit in
  ('del' is a keyword, so can't used as a keyword argument name).

- These map to visible readonly attributes 'fget', 'fset', 'fdel',
  and '__doc__' in the property object.

- fget/fset/fdel weren't discoverable from Python before.

- __doc__ is new, and allows to associate a docstring with a property.