[16.0][ADD] purchase_deposit_company_amount

[AungKoKoLin1997]

This module books a foreign-currency purchase deposit at the company-currency amount actually paid and carries that figure through to the final bill, so the deposit account clears to zero and the exchange-rate difference lands in the inventory cost of the goods instead of being stranded on the balance sheet.

In detail:

• Adds a Company Currency Amount field to the deposit line of a deposit
  bill, which forces the line's balance to the amount entered instead
  of converting the foreign currency amount at the exchange rate.
• Carries the company currency value of a posted deposit bill over to
  the deposit offset line of the final bill, so the deposit account
  closes out at the amount that was actually paid.
• Books the resulting exchange rate difference into the product lines,
  and reflects it in the stock valuation of the received goods.

The field is only available on the deposit line of a deposit bill raised in a foreign currency, and the column is hidden everywhere else. Everything on the final bill is derived from it, and a deposit nobody entered an amount for keeps the standard rate conversion throughout.

@qrtl QT6131

[yostashiro]

Code review and functional test. LGTM. 👍