Calculate estimated and spent time automatically for tickets with task lists

We work around the unavailability of the Epics feature by using task lists to denote parent/child relationships. One of the things that is missing in that model is an update of the estimated/spent time in the "parent" if things change on any issue listed in the task list. I am not sure if that works for Epics but for us it would definitely be good to have because some of use are trying to use parent tickets and task lists to have tickets effectively on different milestones (the ticket itself on milestone A while the parent ticket with ticket A on the task list on milestone B) and we want to have proper time tracking for all of our milestones.

I've not looked closely how we could solve this issue but maybe there is a hook/plugin we can write that could help. The amount of dependent tasks and their open/close status are already tracked automatically, which is good and might provide us some insight on how to bolt the timetracking onto that.

FWIW: This is not to say that those parent tickets should only reflect the time tracking information for their issues in the task list. It should be possible to add additional time spent etc. Just that the figures can't be below the sum of the respective fields of the child issues.

Nested lists should be taken into account as well. :)

@doulget, @gaba, and @sysrqb for visibility as this came up yesterday in during work on label clean-up.