Payment reference now only opens a new window when it is absolutely necessary

Payment methods can now define whether they interrupt the request. Based on that, any context (such as payment method selectors) can choose to only open a new window for those payments methods if payment execution cannot take place in the current window. This is a change from 7.x-1.x, where the payment reference field opened a new window for any payment method, regardless of whether its payment execution would interrupt the request or not.