Add tax to billed hours

Following the discussion on this thread :
https://lists.apache.org/thread.html/ff183c2f074bc35479a07b5acf80b58721d66b8b8c6b753f97f755aa@%3Cuser.ofbiz.apache.org%3E

To sum up : When creating an invoice from a project (from thje Scrum or Project component), we can't apply any taxes because there is no Product associated to the invoice item (the billied hours).

The main solution aims to link the WorkEffort, on which the hours are spent, to a Product by using the entity WorkEffortGoodSatandard.

This Jira can be used to improve/complete the solution and to pass from conception step to implementation.