[438734] Use GEF4 Graph instead of GEF4 Layout to identify predecessors/successors.
[438734] Rename "subgraph" to "pruning" within Zest.FX.

Pruning nodes is no longer associated with subgraphs. A subgraph provides a
unified visualization for a number of (hidden) nodes. Whereas pruning just hides
nodes (and edges), and can give an indication of the hidden content.