Set "RELATED-TO" for completed recurring todos

[WhyNotHugo]

When a recurring todo is completed, we create a copy of it. The old copy is marked as done, and the new copy keeps the RRULE and it set for the next recurrance.

However, nothing really relates these events. Let's use the RELATED-TO attribute, which makes perfect sense for this.

[ngortheone]

I think RELATED-DO might not be the best option for this. There is a conflict with https://github.com/pimutils/todoman/issues/396

For example, tasks that I migrated from Akonadi (KDE) use RELATED-TO to establish task hierarchy

[WhyNotHugo]

This was implemented in 2017.

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From rfc5545#section-3.8.4.5:

By default, the property value points to another calendar component that has a PARENT relationship to the referencing object. The "RELTYPE" property parameter is used to either explicitly state the default PARENT relationship type to the referenced calendar component or to override the default PARENT relationship type and specify either a CHILD or SIBLING relationship. The PARENT relationship indicates that the calendar component is a subordinate of the referenced calendar component. The CHILD relationship indicates that the calendar component is a superior of the referenced calendar component. The SIBLING relationship indicates that the calendar component is a peer of the referenced calendar component.

To avoid confusion, I think it would be sane to add RELTYPEfor recurring relationships, specifically, RELATED-TO;RELTYPE=SIBLING:XYZ, where XZY is the UID of the other recurring instance... though maybe an X- attribute would make sense?

I'll try and consult what other task applications do and try to be consistent.

@ngortheone Can you confirm that Akonadi sets the RELTYPE=PARENT`? If not, it definitely should.

[ngortheone]

Hi @WhyNotHugo I am not using KDE stack anymore, I can't verify easily.