Bug 10613: calculate the GST rate correctly on the invoice page.

Bug 10613: calculate the GST rate correctly on the invoice page.

Test plan:
Defined a GST rate on creating an order, receive it and check that all
prices are correctly calculated.
/!\ Behavior change function of supplier parameters (Include/Don't
include tax for list prices and invoice prices)

Notes: patch tested with Bug 11755 applied first; confirmed that:
- price calculations are correct for all combinations of
  listincgst/invoiceincgst settings in the vendor record
- unitprice (aka "Actual cost") is taken into account on the
  invoice page instead of rrp/ecost, like it should.

Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>