Move in draft state by default

Let the generated move in draft on invoice and bank statement validation. The main reason is to ease the user's day-to-day work. At first we used account_cancel, but this module allow to cancel posted move and that's not allowed.

Two needs here:

• We want to be able to cancel an invoice (as soon as move are not validated) without making a refund. Posted move can't be canceled.
• We want to ensure all moves generated from bank statements and invoices are generated in draft state so we can still change them if needed.

Use this module with account_constraints (https://launchpad.net/account-financial-tools or in http://apps.openerp.com) and you'll get closely the same feature as account_cancel, but with the insurance that user won't change posted move.

The new framework will then be: always work with draft moves, allowing people to change what they want. At the end of the period, validate all moves. Until then, you ensure no-one can change something (or they'll need to make a refund).