Added MemberNodes to represent an object's tree in the PDG.

* JSysPDG: inserts IO and Member nodes before generating data dependencies.
* ObjectTree is now a real tree.
* ObjectTree has string and node iterables.
* VariableAction can be cloned with #createCopy(), the graphNode is set to null.
* VariableAction.Movable can't have object tree, getter obtains inner's.