Cancel the transition to the cash register screen

Starting with API V7Preview5, it became possible to prohibit transition to the checkout screen.

A notification is triggered before going to the checkout screen NavigatingToPaymentScreen. Previously, it allowed you to change your order via IOperationService, available in notification arguments, for example, add payment. It also allowed interaction with the user via IViewManager, show different windows in SyrvePOS.

Now, in addition to everything else, we have added the ability to cancel the transition to the cash register screen by generating an OperationCanceledException exception in the corresponding subscriber. This may be required in cases where additional conditions are checked, the failure of which may prevent navigation to the checkout.