Add overload for QMetaObject::invokeMethod to return a QFuture for the result of the method

I see the work that has been done to make QFuture and signals/slots more cohesive. It would be nice to do something similar for QMetaObject::invokeMethod. Specifically, it would be helpful to have an overload of QMetaObject::invokeMethod that instead provides a QFuture for the result of the invoked method. This would allow writing classes that have public functions that utilize invokeMethod to call the underlying private functions. These public functions could either have a signal connected to them or another class/object (even in another thread) could safely call these public functions directly. This would help to further make the asynchronous interactions in Qt more flexible to fit different scenarios.