cubicweb #3331207 support UNION + ORERBY [open]

playing with an rql syntax tree, one can::

rqlst.wrap_selects()
subquery = rqlst.children[0]
subquery.add_sort_var(subquery.children[3])

but this is just a workaround.

prioritynormal
typeenhancement
done in<not specified>
load1.000
load left1.000
closed by<not specified>