- Incorporate cost of any sorting of the result set for windowing, i.e.
  we have an ORDER BY and/or PARTITION BY in the window specification

- Add this information to JSON explain

- Fix a bug in JSON explain for windowing: we only printed the
  effective sort key, i.e. the concatenation of any PARTITION BY and
  any ORDER BY, if we had an ORDER BY, not for solely PARTITION
  BY. (This fix has nothing to do with costing, but was discovered
  while studying the explain outputs.)

- Fixed a bug where before, the cost of a final ORDER BY would get
  wrongly attributed the the windowing, e.g. for

  EXPLAIN FORMAT=JSON SELECT i, j, SUM(i+j) OVER (ROWS UNBOUNDED PRECEDING)
foo FROM t ORDER BY foo;

  In this case, the windowing has no ordering.

- Added new explain tests