Fix issue with qtawesome fonts not rendering on OS X

[SylvainCorlay]

Fixes #2597.

[SylvainCorlay]

Although, icons are on my retina display in the toolbars and push buttons are pixelated, while they are perfectly crisp elsewhere like in the preference menu.

The two clues I have from earlier research is that

• starting with Qt5, we should be able to set app.setAttribute(QtCore.Qt.AA_UseHighDpiPixmaps) at the level of the QApplication.
• besides, it some threads mention that the python.app's Info.plist should be edited to have

<key>NSPrincipalClass</key>
<string>NSApplication</string>
<key>NSHighResolutionCapable</key>
<true/> 

[ccordoba12]

Thanks @SylvainCorlay, this really fixes the problem :-)