The index was there with the right name, but indexing cost_type_id

- Now showing timesheet costs as closed by default

  from which the current document was created

- In invoices/view.tcl showing the related customer purchase orders
- Allowing to promote a normal invoice to a timesheet invoice, so that
  the report period is shown

- Disabled anything except Finish-to-Start dependency in
  the GanttSchedulingController
- Still need to adapt display in GanttBarPanel