Base invoice posted status on the presence of a posted transaction rather than a posted account

Bills and invoices that are posted and subsequently unposted again still store their
posted account internally as a convenience to the user (upon reposting the old
account will be offered by default) so it's not a reliable test for the posted state.
The posted transaction on the other hand is guaranteed to only exist when the invoice is
posted. This should fix a slew of small and perhaps larger side effects, such as
a posted bill still appearing as editable, critical warnings when creating new bills/invoices
and so on.

libgnucash/engine/gncInvoice.c

@@ -1859,7 +1859,7 @@ gncInvoiceApplyPayment (const GncInvoice *invoice, Transaction *txn,
 gboolean gncInvoiceIsPosted (const GncInvoice *invoice)
 {
     if (!invoice) return FALSE;
-    return GNC_IS_ACCOUNT(gncInvoiceGetPostedAcc(invoice));
+    return GNC_IS_TRANSACTION(gncInvoiceGetPostedTxn(invoice));
 }
 
 gboolean gncInvoiceIsPaid (const GncInvoice *invoice)