Fixing printing without posting in SQL-Ledger

In official SQL-Ledger, it is possible to add an invoice or a POS transaction without posting it and then forgetting to post it unintentional or sometimes, intentionally, by user to commit fraud.

I have made few changes in SQL-Ledger code which prevent this:

1. An invoice cannot be printed if it is not posted already.
2. As much invoice lines are printed as are already posted.

Here are these code changes which you can also do in your SQL-Ledger installation to achieve this.

I intend to make few more changes to avoid the situation where user makes some changes to the invoice, prints it and then skips posting it. I have marked this plan with TODO in these code diffs.

If you have any further ideas or queries on this topic, you can share them in comments.