Relative threshold UI

[dperelman]

My understanding of the semantics of threshold dates for recurring TODOs is that the threshold is treated as a number of days before the due date when creating the new TODO. It would be nice if there were an option to have the TODO creation GUI display (and allow editing) the threshold in terms of days (or weeks/months) instead of as an absolute date.

(Allowing thresholds to be relative in terms of months instead of days would also be nice, but involves changing the format, so is out-of-scope for this request.)

When creating TODOs I usually (although not always) think of the threshold in terms of how long before the due date I want to be notified of it, but then have to manually count out what day that corresponds to. This is a minor annoyance, but it is a bit more of an annoyance when creating multiple tasks with different due dates but the thresholds the same number of days before that due date.

This could either be a setting or there could be a checkbox on the create TODO dialog of whether the threshold should be relative. In relative mode, it should look like the recurrence UI. This wouldn't change the format: in relative mode, the actual text line generated would still have an absolute date after t:.