iOS 13 support

• The project has been updated to support iOS 13 and Xcode 11.
• Dark mode is supported on iOS 13 and above. A new userInterfaceStyle property for SRGLetterboxView lets you choose whether the appearance is controlled by the system, or whether the track appearance should always be light or dark. Note that this setting only affects part of the UI elements associated with Letterbox (track selection only at the moment).

Remark: Transitive dependency on SRGLogger has been updated. As a result your project might not compile anymore. To fix this:

• If your project was not using SRGLogger_Swift.framework, it will still compile fine after updating to 1.14.4. Nothing needs to be done.
• If your project was referencing SRGLogger_Swift.framewok but is not using it actively, you can simply remove this dependency from your project to have it compile again.
• If your project was using SRGLogger_Swift.framework, add srglogger-swift-ios as dependency to your Cartfile, and update your dependencies to have your project compile again.