OS X Retina support

By adding <key>NSPrincipalClass</key> <string>NSApplication</string> Into the Info.plist of the SoulseekQt.app bundle, I got the text and images in the MacBook Pro Retina to not look blurry. Apple docs: http://developer.apple.com/library/mac/#documentation/GraphicsAnimation/Conceptual/HighResolutionOSX/Explained/Explained.html#//apple_ref/doc/uid/TP40012302-CH4-SW7 A forum thread on the same issue for Qt Creator: http://www.qtcentre.org/threads/50130-Qt-Creator-on-MacBook-Pro-Retina