fix: deleting the date a reminder is related to should delete the reminder

[raimund-schluessler]

          @jaylinski We should stay in accordance to the CalDAV specification, which states that relative alarms without the date the alarm is related to are not allowed. See the description in https://icalendar.org/iCalendar-RFC-5545/3-6-6-alarm-component.html:

In an alarm set to trigger on the "START" of an event or to-do,
      the "DTSTART" property MUST be present in the associated event or
      to-do.  In an alarm in a "VEVENT" calendar component set to
      trigger on the "END" of the event, either the "DTEND" property
      MUST be present, or the "DTSTART" and "DURATION" properties MUST
      both be present.  In an alarm in a "VTODO" calendar component set
      to trigger on the "END" of the to-do, either the "DUE" property
      MUST be present, or the "DTSTART" and "DURATION" properties MUST
      both be present.

So, the options 2 and 3 in your list are not possible by the specification alone. Furthermore, the user would still see the alarms being present, and might falsely assume that they are still working, where in reality they won't.

However, we are free to convert relative alarms to absolute alarms, if the related date is deleted. We could present the user with a popup to let him decide whether he wants to:

• cancel the deletion of due or start date
• convert relative alarms related to the deleted date to absolute alarms
• delete the relative alarms related to the deleted date

The other option would be to not confront the user with this problem, and simply convert relative to absolute alarms. This is what we mostly do, to prevent pop-ups and dialogs as they often interfere with the workflow.

@jancborchardt and @nimishavijay might also have an opinion here.

Originally posted by @raimund-schluessler in #2743 (comment)

[raimund-schluessler]

Completely agreed! Relative dates make more sense anyway, and for the deletion popup I would suggest that we show it but simplify the wording to

This task has 2 alarms. Would you like to keep them?
[Keep alarms] [Discard alarms] and [Cancel]

That way the user won't be confused when the alarm that they had set with the due date in mind goes off without any warning :)

Originally posted by @nimishavijay in #2743 (comment)