[trunk,6.0] supplier invoice, default price suggested should exclude VAT

Steps to reproduce the bug using web client:
1) Create new Supplier Invoice
2) Select any partner/address
3) Enter total amount for the supplier invoice
4) Click new invoice line button
Note: the unit price suggested for the line is the same as the total amount of the invoice
5) Select a product which has a default VAT code set (for example 5% in my case)
6) The unit price suggested for the line stays the same as the total amount of the invoice

Expected:
For 6) the unit price suggested changes to be (Total_invoice - Total_vat) / qty

Attaching a branch to fix this.

Applicable on v6.0 and trunk.

This change greatly speeds up encoding supplier invoices.