Use incremental sort paths for window functions

Over on [1] someone was asking about chained window paths making use
of already partially sorted input. (The thread is on -general, so I
guessed they're not using PG13.)

However, On checking PG13 to see if incremental sort would help their
case, I saw it didn't. Looking at the code I saw that
create_window_paths() and create_one_window_path() don't make any use
of incremental sort paths.

I quickly put together the attached. It's only about 15 mins of work,
but it seems worth looking at a bit more for some future commitfest.
Yeah, I'll need to add some tests as I see nothing failed by changing
this.

I'll just park this here until then so I don't forget.

David