Qt Purchasing: Add finished signals when registering a product and restoring purchased items.

That would be useful to known when the registering and restore process finished.
Actually the registration get productRegistered / productUnknown, and we have to connect both signal to check process success.
Will be fine to have a final productRegistrationFinished(productId, bool success);
The restorePurchase() also is a bit limited - we can't known when the process is finished, meaning we can start the restore but don't known when it finished.
Again, here a restorePurchasesFinished() will be welcome.
I did checked iOS/OSX and Android do get informations about finished restore purchases.
See https://developer.apple.com/library/mac/documentation/StoreKit/Reference/SKPaymentTransactionObserver_Protocol/index.html#//apple_ref/occ/intfm/SKPaymentTransactionObserver/paymentQueueRestoreCompletedTransactionsFinished: for OSX.
Those signal would also help a bit when some people would want to use QtIAP synchronously.