New KCalendarCore custom property API
Problem with the current solution:
- No access to property parameters
- Forces X-KDE- prefix or use of awkward "nonKDECustomProperty" API
- No support for property types other than strings
Suggested solution is returning a new Property type instead of a QString, which has access to the content as well as the parameters. There is a backward compatibility challenge with this though:
- a new property only accessible by name becoming available as proper API will then suddenly no longer be accessible by name
- an unsupported property or property parameter type will be reported as string, but change to the real type when support is added
Both scenarios break existing user code.
Conclusion: problem already exists now, we have no good way to avoid that, so accept this can happen.
There's initial work on this in https://invent.kde.org/vkrause/kcalendarcore/-/tree/work/vkrause/new-property-api.