macosx: Upgrade xib files from IB 3 to IB 4-6 format

This makes usage of the old Xcode 3 and Interface Builder 3 obsolete.
All further changes need to happen in Xcode >= 4.

BGHUDAppkit elements were preserved, but for further changes the respective
subclass for the views need to be set manually.

Also resyncs all xibs with the source files.