Specify the behaviour of StopTimeEvent on different schedule relationships.

[miklcct]

Fixes #490

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[gcamp]

I'm not a fan of fields changing meaning depending on the context, if we would add a way to specify schedule time I would prefer having a separate field.

In addition, I'm sure you've seen that the usage of ADDED has been discouraged. I would think defining clearly ADDED would be useful before adding more capabilities to it.

[leonardehrenfried]

I agree. Clarifying ADDED has the highest priority IMO.

[miklcct]

I am going to work on another PR for specifying ADDED (and REPLACEMENT) types.