Add a start date to tasks / enhanced Today tab

[bestlinuxgamers]

Is your feature request related to a problem? Please describe.
I really like the "Today" tab. It's a great overview of what I have to do for the day. However, it only shows tasks that are actually due on that day. Not every task has to be completed on the day it is due, in fact it may even be too late by then.

For instance, consider a homework assignment with a specific due date. Ideally one would want to complete the homework on some day the week before the due date. While it's possible to schedule the task for an earlier date, doing so removes the actual due date from view, making it harder to reschedule and handle the task.

Describe the solution you'd like
I would love to be able to set a second "start date" which would precede the due date. In the period between the start and due date, the task could appear in a separate section of the "Today" tab, below tasks that are due for the day. This would create a comprehensive daily to-do list, including tasks due on the current day and suggested tasks with later due dates that should ideally be started earlier.

Describe alternatives you've considered
The current implementation of filters it not powerful enough to resolve this problem, because the timeframe to work on each task varies and this information is not currently saved. Also there is no page containing all tasks to which the filter could be applied.

Additional context
This is similar to #740, but I think the usecase is much different and thus deserves a separate issue.