[7.0][addons] Misleading incoherences with boolean field "sent" in account addon

[Romain Deheele - Camptocamp]

Hello,

Here are two misleading incoherences about boolean field "sent" in an invoice:
In account addon,
1)When you print an invoice, the boolean field "sent" is set to True.
It's really misleading because near the print button, you have an other button to send the invoice by mail.
At first sight, we really don't expect that it's the print button which set this field to True.

2) Then, if you cancel the invoice and you set it to draft, here, we could imagine that this field "sent" is set to False, but it's not the case! :(

Thanks in advance for your return,

Romain