Stability fixes

• generate_appcast option to read private key directly from the keychain (Tamás Lustyik)
• Add delegate callbacks for finished download and extraction related events (Csaba Horony)
• Don't check for updates if Do Not Disturb is on (Kornel)
• Expose CodesigningVerifier, add codesign info API (sunuslee)
• Threading fixes:
  • Fix potential hang with dispatch_sync to main thread (Brian Bergstrand)
  • Fix closeCheckingWindow called from background thread (Alexey Martemyanov)
• Improve 'read-only' error message (#1192) (Adrian Thomas)
• New Spanish localisation (Ken Arroyo Ohori)
• Updated Finnish language resources (Jason Pollack)
• Hungarian localization (Csaba Horony)
• Log more information about authentication requests (Kornel)
• Explicitly specify types to silence "Messaging unqualified id" warning that's new in Xcode 10. Removed __has_feature(objc_generics) check and use generisc to help silence the warnings. (Kent Sutherland)
• Fix binary delta creation on network drives (sagecook)
• Fix compilation issues on Xcode 10 with new build system (Leo Natan)