use a new arg identifier winset for window functions.

use a new arg identifier winset for window functions.
Update window functions to be flagged as taking input geometry winset
Update templste.xml (to instruct how to flag window and aggregate functions)
Update the comments generation to handle the new geometry winset type arg
Closes #3474
Mark ST_ClusterWithin and ST_ClusterIntersecting
as taking geometry set (so flagged as aggregates)
Closes #3476

git-svn-id: http://svn.osgeo.org/postgis/trunk@14711 b70326c6-7e19-0410-871a-916f4a2858ee