Enhance Graph.java's Subgraph object toString() method to show recursive node structure and properties.

While developing DdlSequencer tests, a need developed obtain a sub-graph of the sequenced DDL file and operate on the resultant nodes and properties. However, the Sub-graph was difficult to debug because viewing child nodes and properties were difficult to see in values view.

By constructing a more descriptive/recursive toString() value for subgraph, it would make debugging and testing much easier.