[IMP] sale_timesheet: introduce timesheet revenue

This commit introduces the concept of timesheet
revenue, aka how much the timesheet bring back
money.
If the timesheet is not invoiced yet, the computed
revenue is theorical, based on the billable type
(if your product is Time and Material, or Fixed Price).

The formulae use at the timesheet line creation is

     If product on delivered quantity:
        timesheet hours * (SO Line Price) * (1- discount),
     elif product on ordered quantities & create task:
         min (
            timesheet hours * (SO Line unit price) * (1- discount),
            TOTAL SO - TOTAL INVOICED - sum(timesheet
                revenues with invoice_id=False)
         )
     else: (non billable case)
        0

When the timesheet is invoiced, the revenue is recomputed
based on the prorata of the invoice amount : prorata of
number of hours for 'time and material' product,
prorata of the theorical revenue for 'fixed price' product.

The timesheet revenue is expressed in the currency of the
company (Monetary field, based on the company_currency_id field
of account.analytic.line) to ease reporting.
This feature normally can handle multi currency (different currency
from SO, INV, and analytic account).