Added Box setNodeMetadata() and getNodeMetadata() methods.

These will allow the specification of per-instance node metadata for Boxes, in the same manner as the existing per-instance plug metadata.
Fixed Metadata signal bindings to not expose InternedStrings.

These are useless in Python, so we use standard python strings instead.
Made NameWidget accept None for the the GraphComponent.

It can be convenient to construct one with None, and then call setGraphComponent() later.