Add setting to disable adding completion date

[Lezurex]

Feature Request

What problem does it solve?

In PR #885 we had a deeper look at how completion and creation dates have to be set in todo.txt. Currently, sleek automatically adds the completion date when a task is completed. However, some users might not want this.

How does the user interface change to accommodate this request?

A toggle in the settings should be added to enable/disable automatically adding the completion date. However, we can only omit it if there's no creation date. If there's a creation date, we always have to add the completion date so that the two dates can be distinguished from each other.

[ransome1]

we always have to add the completion date so that the two dates can be distinguished from each other.

@Lezurex I am implementing this right now. However I notice a usability issue: a user who never uses creation dates might wonder why, with the append completion date setting enabled, a completion date is never added to the todo upon completion. I am unsure how to solve this as simply adding the completion date without a creation date, as you observed, would cause the completion date to be treated as the creation date, breaking the semantic logic. Imo the new setting will feel like a bug to users. What are your thoughts on this?

[Lezurex]

Hmm yeah it's difficult. Actually, it might not even be allowed to not set the completion date. Rule 2 in the spec states the following:

With the completed date (required), if you've used the prepended date (optional), you can calculate how many days it took to complete a task.

On the other hand, the image in the spec says the completion date is optional. But that might only be because it's not required for incomplete tasks.