New downloader

• Refactoring of downloader code to avoid deprecated methods (Deadpikle)
• Changes to which methods run on the main thread.
  Note: some delegate methods may be called on non-main thread now. (Kornel)
• Update Japanese localization (1024jp)
• Update Sparkle.strings (Stefan Paychère, Adrian Thomas)
• Fix Sparkle clients polling too frequently (Jonathan Bullard)
• Handle SecTransformExecute errors (Kornel)
• Silence Touch Bar availability warnings on Xcode 9 by using API_AVAILABLE. Disable gnu-zero-variadic-macro-arguments to prevent warnings from use of API_AVAILABLE. (Kent Sutherland)
• 10.11 SDK compatibility (David Fuhrmann)