Added ability to attach customer or supplier to any tax transaction

[lubos]

@Patch

• Receipt: all line items tax is of type sales (independent of the sign of the line item amount), and can be linked to a single customer if desired
• Payment: all line items tax is of type purchase (independent of the sign of the line item amount), and can be linked to a single supplier if desired

This is being covered by this topic. Working on it.

Improvement to payment and receipt forms

It would be very helpful if the cash payment and receipt forms were improved to include several features: Linkage to customer/supplier subaccounts. Established customers don’t always buy on credit, making a sales invoice unnecessary. Likewise, businesses often pay cash to established suppliers, so purchase invoices are not needed. This linkage would eliminate separate invoice and spend/receive transactions. And it would allow the complete history of a customer/supplier in a statement, not jus…