Add options on task dependency

[lredor]

The dependencies between tasks are very limited and are currently used only for graphical purposes.

If could be interesting to have the same dependency options as in GanttProject:

• Type of dependency:
  • End-to-Start
  • End-to-End
  • Start-to-End
  • Start-to-Start
• Delay: The lag between the start or finish of the predecessor task and the start or finish of the current task (depending on the type of dependency).

This is a critical blocker for using Pepper effectively.

More explanation on the GanttProject documentation: https://docs.ganttproject.biz/user/scheduler/index.html#dependency-constraints