Add a feature to make start date retain its value

[S5E5S]

Recently I was reporting a bug that turns out to be a feature in #2089.

Although this feature was useful, this will become a problem in case we want to retain the start date.

In my case, sometimes I'm using the start date to keep track on when did I start a task and how much time has passed since then, so the start date should be fixed.

But for due date, it mostly change, especially for tasks that wasn't a school or business work.

With that considered, can you add a feature where start date wasn't going to change if due date is changed? Maybe a checkbox that offer to retain the start date inside a start date date picker?

Thanks :)

[ghost]

As a side point to this: start dates in the past probably should not be red. Because it's the start of a task, it's not "overdue"/red when the start date is passed. But I guess there could be differing opinions about this