To enable burn down charts, users should report the remaining time per task in the iteration (version). This field should be independent and should be different that the estimated time.

Typically, the estimated time is only a 'first estimate', and we need it to keep as such. However, the 'Remaining time' should be updated regularly (every day) so that to easily draw a burn down chart.

This feature is to introduce the 'Remaining time' concept along with a facility to easily report the remaining time (similar to how users currently easily report their spent time). This cannot be simple introduced using custom fields, because this is not just a field. It will have some more side effects, like accumulating this field on parent tasks, and probably on the version as well.