Allow visualizer assignment to propagate through Sink group nodes

[glopesdev]

Currently type visualizers are associated with either data types or operator types. Operator-type specific visualizers are automatically propagated through subject nodes, disabled nodes or empty group nodes, but not within any other reactive operators.

This proposal is to extend this implicit propagation specifically to Sink group nodes. Sink nodes implicitly create an internal subject to allow their nested logic to be handled without changing the original sequence. Implicit propagation of visualizer assignments through Sink nodes would allow for example simple transformation of GUI element notifications before multicasting the data through subjects elsewhere in the workflow.