Thai Localization - Withholding Tax (multi taxes)

This module extends the payment registration screen so that Thai withholding tax can be handled for several invoices at once.

• On the payment form, when you choose the option to reconcile with multiple deductions, Odoo looks at the invoices you selected and, for every invoice line that has a withholding tax set, it creates a deduction line automatically. The deduction amount is calculated from the withholding tax percentage applied to the line subtotal.
• Each deduction line now shows a "WT" column (Withholding Tax) next to the account, so you can see which withholding tax each deduction belongs to.
• When you pick a withholding tax on a deduction line, the account and the label of that deduction are filled in automatically from the withholding tax configuration.
• If you mark a deduction line as "open" (meaning you enter it manually), the withholding tax link is cleared, so manual deductions stay manual.
• The withholding tax is also carried over to the accounting move line created for the deduction, keeping the link between the payment and the tax.

It works together with the Thai withholding tax module and the multiple deduction payment module, and it affects the account.payment and account.payment.deduction models.