Add QStateMachine constructor that takes a ChildMode

Add QStateMachine constructor that takes a ChildMode

Back when QStateMachine was changed to inherit QState, this
constructor was conveniently left out because setting the state
machine (root state) to be a parallel state group didn't actually
work. But as of commit d281aa6936ad01e28dacabb41bd9eb59891f85a1,
it does work, so add the missing constructor.

Task-number: QTBUG-15430
Change-Id: I68c599baa0ef1bfc869195140cf5daf645e75b8b
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>