Add support for "Depends on" feature for tasks

[PierreRaybaut]

• Display tasks number in the tree view (in a new column?)
• Update tasks number when necessary, so that it always reflects the actual order of the tasks
• Add an option in the tree view (maybe not an additional column) to allow the user to specify the dependencies of any task

[PierreRaybaut]

Done in v1.6.0