Adding support for Apple's Retina display for fonts

Enable improved rendering of fonts on Apple's retina displays by
adding a custom bundle plist file with the following additional
elements:

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

The remainder of the input plist file is based on the current in file
from CMake.