InAppStore: no support for paymentQueueRestoreCompletedTransactionsFinished

[BZ1234567890]

There is currently no way of knowing when the "restore purchases" operation completed if there are no purchases to restore (e.g., if player never bought any IAPs) or if an error occurred during the restore operation.

That is slightly annoying if you have GUI shown during the restore operation (for example, a progress bar or a spinner), as you don't know when to close it.

It would be nice if InAppStore had support for the paymentQueueRestoreCompletedTransactionsFinished: and paymentQueue:restoreCompletedTransactionsFailedWithError: events which get triggered in the SKPaymentTransactionObserver when restore operation completes or fails. This would provide us with more control over the restore workflow.

• https://developer.apple.com/documentation/storekit/skpaymenttransactionobserver/1506063-paymentqueue?language=objc
• https://developer.apple.com/documentation/storekit/skpaymenttransactionobserver/1506101-paymentqueuerestorecompletedtran?language=objc

[BZ1234567890]

I see that this pull request would close this issue:
#42