Added support for price tiers

When editing a variant's price, the Merchant Center now supports management of price tiers. A price tier is selected instead of the default price when a certain quantity of the ProductVariant is added to a cart and ordered. For example: the price can be lower if more than 10 items are ordered.

A price tier is always defined by a

• minimum quantity (the minimum quantity this price tier is valid for)and
• a (monetary) value.

Price tiers are also shown in the variant's price list view.