Minor improvements to qt frontend on Mac OS X.

* Fix build to run with older macosx SDK.
* MacOSX10.8.sdk is now required at this time (the QT installation package was apparently built with the 10.8 SDK. Note the 10.9 SDK is the only one available with Xcode 6, so you'll need to extract the old 10.8 SDK from Xcode 5.1.1).
* Use a custom Info.plist to prevent SafeDisk from showing up in the Dock.
* Make sure that a clicking OK on a message box doesn't cause the app to quit (due to last visible window being closed).