set the original time estimate when creating subtasks

[Maximilian Hantsch-Köller]

As a user of the taskmaster plugin, I want to be able to set the original estimate for each of the created subtasks, so that I can prevent the extra work of going back to each created subtask individually to add that information.

Ideally, the subtask list in the taskmaster plugin has an additional column for the time estimate.

Ideally, this works together with issue #2 and can be pre-populated from a template for each of the subtasks in the template.