Add "end date" functionality to tasks.

The concept of “end date” is the one of a softer version of “due dates”. While “due dates” have a specific deadline that should not be missed, “end dates” implies meeting a specific deadline is just a nice to have.

The advantage is that it allows an automatic sorting of tasks by degree or urgency:

• Tasks with (approaching) due dates can be focused on first

• Tasks with (approaching) end dates may be selected as next in line,

• etc.

Hence, it would add an interesting degree of granularity between tasks which have a due dates and the the ones that do not.