Sort tasks by due date

[iafan]

Right now, we use a combination of due date + number of incomplete tasks to determine the task "importance" (which affects its color, from grey to red), and also sort the task based on this importance.

This has proven to be a bit misleading with the large number of tasks, since large tasks can overshadow the smaller ones that need to be finished sooner.

I suggest to keep the "importance" for coloring purposes only, but sort tasks by due date first, and within tasks with the same due date, sort them by the project name/file path (the latter is just to stabilize the output).