Window(order_by) should allow usage of descending string syntax to be used

[Simon Charette]

The QuerySet.order_by and ​some aggregates ordering kwarg allows for the leading dash syntax to be used but Window.order_by doesn't as it solely wraps the provided order_by in ExpressionList(expressions=order_by).

This makes for an inconsistent API so I suggest we reuse the logic in OrderableAggMixin.__init__ in Window.__init__

As a related note it seems most of the logic of OrderableAggMixin could be simplified by using ExpressionList.

It's a shame that we used ordering and not order_by as a kwarg for OrderableAggMixin as it's now inconsistent. Also not sure how much of a public API the OrderBy expression is but I wish it was initially named Sort (or Ordering?) so that we could define

class OrderBy(ExpressionList):
    template = 'ORDER BY %(expressions)'

    def __init__(self, *expressions, *extra):
        expressions = [
            (Sort(F(o[1:]), descending=True) if isinstance(o, str) and o[0] == '-' else o)
            for o in expressions
        ]
        super().__init__(*expressions, **extra)

And then simply use this abstraction in Window and Postgres orderable aggregates.

Assigning to myself as I plan to have a look at this in next few days.