figure out how to handle dangling weak/path ReferenceOne/Many properties

this is a follow-up of #116

the problem is: what happens when a ReferenceOne path or weak reference property points to something non-existing? is the property null? is the currently invalid reference removed on save or kept (which would make sense, in case the node at path appears later). but if it is usually kept, how can we delete it if we want?

Start by reading follow-up issue #116 and tracing the ReferenceOne/Many path and weak-reference behavior described here. Compare what happens when the target is missing, on save, and on explicit deletion; done means a decided, documented behavior for dangling references and a clear way to remove one when desired.

Written by the indexing model from the issue text.