Location for window function API definitions

The window functions patch defines some API functions and macros for
window functions.  These are currently defined in executor/nodeWindow.h,
but that seems to me like a fairly inappropriate location.  The executor
main body, which would be the normal consumer of that include file, has
no interest in the API functions; and conversely the window-plan-node
functions that are defined for the executor are not things for window
functions to call.  So I think the API definitions need to go someplace
else.

The only existing file that seems appropriate is include/funcapi.h, but
that also contains a lot of stuff that wouldn't be useful to a typical
window function.

If we invent a new include file for the purpose, possible names might
beinclude/windowapi.hinclude/executor/windowapi.h
or something else.

Comments?
        regards, tom lane