Added the NSPrincipalClass key to our Info.plist file to enable high DPI on OS X.

High DPI is not supported in Qt 5.0.2, but it should be in Qt 5.1.
Still, by adding this key, we will be ready. There is another key
(NSHighResolutionCapable), but it's optional and set to true by
default, so we don't need it. For more information, see
http://blog.qt.digia.com/blog/2013/04/25/retina-display-support-for-mac-
os-ios-and-x11/.