Add dependencies between tickets

[hjp@…]

A dependency "this ticket depends on tickets a, b, …" should be added.

1. Sometimes a task can only be started once another task is completed.

1. Sometimes a task can be split into subtasks. Completion of the task then depends on completion of its subtasks.

A TODO list could then either only show tickets which don't depend on other tickets or sort tickets by dependencies.

[Emmanuel Blot]

This is a diplicate of #31

[Christian Boos]

Right, this is an often requested feature.

I already had an implementation for this, in an older xref-branch, and it is still somewhat testable there trac-obj-branch, though this is very outdated w.r.t. the trunk and not exactly how I'd like to redo it.

You are right to point out those two common use cases for ticket dependencies.

I planned to support the second scenario by the way of a Create sub-ticket in the ticket view, that would automatically prepare a new ticket with the appropriate defaults and correct linking to its parent ticket.

I'll keep note of the TODO list idea.