Fix issue on OS X where QMainWindow is hidden on startup.

The QMainWindow is hidden at startup on OS X as a side effect of adding a QToolBar. Adding a hint when initializing the QMainWindow is a workaround for this issue. See issue description at https://bugreports.qt-project.org/browse/QTBUG-4300.

Calling show() on QMainWindow after adding the toolbar is another possible workaround.