Make setting due date today an option in context menu

[syvb]

This is useful for when you want to reschedule future tasks to today, or bump forward overdue tasks to today.

[nekohayo]

Hi, had you seen issue #550? There's some nuances in there, where I thought, regarding the start dates:

"Today" [as a start date] makes little sense when you have "Clear Date" and would suggest removing it (and the separator that follows it) [from the popover menu from the headerbar] for consistency [with the right-click menus].

For the due dates however, I agree that "Today" is missing from the right-click menu. I didn't think it would be a huge deal when I saw it being removed last year, but I can definitely say that in practice it happens more often than not that I want to quickly retarget the due date of a task to today...

[syvb]

@nekohayo I've updated the PR to just change the due date menu

[Neui]

Looks ok code-wise and works.

[diegogangl]

Merged, thanks!