[IMP] sale_timesheet: invoice from plan project

We want to invoice from the project overview. To do so,
a button can help people: it is only visible when an order
linked to task in that project has something to invoice
Any sale order line, even the ones with no service product
will be invoiced.

However, the user will have to choose which SO (involved in
the project) to invoice. Then the "invoice order" wizard
takes over.