Qml2Puppet: Use helper functions from Qt 5.6

In Qt 5.6 we do not need to access the private API directly.
QmlPrivateGate is forwarded to the helper functions in Qt.

This removes direct dependencies to QML internals in qml2puppet.

Change-Id: I274cb306815824c988b6d79966f007298f6d1e60
(cherry picked from commit d6d0bd60357a41cd6c83b511fd30ab12157ed95d)
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>