Update of price in offers on store group level (RTC-24361)

When an item with store price is added to an offer on store group level, and then a store group price is added and item selection is reloaded, ordinary price is updated with correct store group price. When the new price is higher than the previous, the promotion price is not updated. When the new price is lower than the previous, the promotion price is set to ordinary price.
In case of promotion on all stores or store group, when an item has prices only in stores, price in store with smallest number is used when adding price to offer (store numbers sorted as text).
In 'All stores' offers, the price is selected in hierarchy: profile, price zone, store.

Documentation (RTC-24949)

DayPeriod (timeFrom and timeTo fields) is displayed properly in examples of import and export file.