Support custom predefined event reminders that occur after the event
Currently when creating an event and adding reminders, the "Add predefined time" option only allows defining a reminder before the event, since it has no "before/after" option.
This means that if e.g. frequently creating events with a reminder some time after the event (an example would be a reminder at the beginning of working hours to take action on an event that happened overnight), the user has to manually create a custom time for every event.
This should be reasonably easy to fit into the relevant GSettings key, by changing it to a signed value and using negative values for "after the event time".