Enable time-estimation links between issue and sub-issues.

Currently, creating a sub-issue/parent-issue link between two tasks only sets a descriptive link with a string on each task. This is a major problem on our project where we often need to divide issues into subissues, but still retain valid time tracking.

Its important to be able to view all super-tasks of a version, and see how well they have progressed without being bogged down by the details of the subissues.

At the same time it is important to use estimates for the subissues themselves.

Only two approaches are possible (as I see it) in Jira 2.6.1:

1. Let the super-issue reflect the time needed to wrap things up, after the sub-issues are complete. The problem is that one can't view all supertasks to get an estimate of version completion.

2. Let the super-issue-estimate be the sum of all sub-issue-estimates. This causes two problems: Developers have to log work twice; once in the subissue, and once in the superissue, and also, this causes wrong numbers in the time tracking of a version, since all estimates are doubled (both in super-issue and sub-issue)

Neither of these solutions are very usable, so a harder link between tasks is needed, perhaps enabling a hierarchy of tasks.