On partners form view, under the "Sales & Purchases" tab, one can define a
"Delivery schedule preference" for each partner.

Possible configurations are:

* Any time: Do not postpone deliveries
* Fixed time windows: Postpone deliveries to the next preferred time window
* Weekdays: Postpone deliveries to the next weekday

After selecting "Fixed time windows", one can define the preferred delivery
windows in the embedded tree view below.
