This patch implements the further generalization of an idea suggested by Stefan
at http://lists.gnu.org/archive/html/emacs-devel/2013-02/msg00088.html. The main
motivations are 1) make 'struct window' even bit smaller and 2) simplify the code
like:

if (!NILP (w->hchild))
   foo (w->hchild);
else if (!NILP (w->vchild))
   foo (w->vchild);
else /* assume leaf window with w->buffer */
   bar (w->buffer);

to:

if (WINDOWP (w->object))
   foo (w->object);
else /* assume leaf window with buffer at w->object */
   bar (w->object);

As usual, reviews and comments are highly appreciated, especially for the
'struct window' member which replaces hchild/vchild/buffer (I don't like
too generic 'object', but couldn't think up the better name).

Dmitry