Navigator quits Qt applications on PlayBook

With current Qt5/stable branch, applications on PlayBook are terminated by the Navigator shortly after startup, because the root window is not posted to the windowing system. This regression has been introduced by commit 77a06e7e6c517003585ba53b9776e90c8d290e84, which moved the QQnxRootWindow::post() call from the QQnxRootWindow ctor to the QQnxRootWindow::makeTranslucent() method. However the latter is only invoked if a translucent window is requested by the application.