Allow instance definition reuse between grouping instantiations

Currently instance notifications are instantiated for each point where a grouping is instantiated – except their content is completely defined at instantiation point.

This makes it difficult to transplant data from one instantiation to another, as they do not share a common interface (as grouping/dataobjects do).

Update codegen to emit a standalone interface for notifications defined as children of a grouping, so that each instantiation of those notifications does not just redefine the attributes, but rather inherits it from the generated interface.