Create invoice including work report from list of reported work

[scaphilo]

Linked with #119
It shall be possible to create an invoice that contains all spent work on tasks of specific project.
When the list of tasks was created from a salescontact before, the report and the invoice shall contain the same positions as the previous base salescontract.

Example workflow:

• koalixcrm-user creates offer
• customer orders based on the offer
• koalixcrm-user creates purchase confirmation and starts project
• koalixcrm-users (employees) report worked hours onto the available tasks
• koalixcrm-user creates invoice for the customer.
  The created invoice is already filled with the reported work from the employees