Fix default system font issue on Mac Mavericks with Qt-4.x.

See https://bugreports.qt-project.org/browse/QTBUG-32789 for more.

Change-Id: I88552a2204018a2ef662265ad61f9fec54af8860
Merge topic 'FIX_mac-mavericks-font' into master

d5da5394 Fix default system font issue on Mac Mavericks with Qt-4.x.