engine: autogroup, traits, graph: Extend autogroup API

This extends the autogrouping API so that a child can easily get a
reference to the parent that it is autogrouped in. This can simplify the
API for some resources when it makes sense to allow them access to the
parent handle. Use sparingly and intelligently!

engine/copy.go

@@ -93,6 +93,7 @@ func ResCopy(r CopyableRes) (CopyableRes, error) {
 			if !ok {
 				return nil, fmt.Errorf("resource wasn't groupable")
 			}
+			g2.SetParent(dst) // store who my parent is
 			grouped = append(grouped, g2)
 		}
 		dst.SetGroup(grouped)