Fix for QMainWindow hiding QToolBar on OS X

Add the Qt::MacWindowToolBarButtonHint when initializing QMainWindow on
OS X as a workaround for issue of QMainWindow hiding when adding a
QToolBar later. See https://bugreports.qt-project.org/browse/QTBUG-4300
for description of issue.
Merge remote-tracking branch 'rsprouse/master' into playground