Switch to a proper logger

[stackotter]

At the moment SwiftCrossUI prints all of its warnings straight to stdout. We should move to using a proper logger so that we can easily re-route SwiftCrossUI's output to wherever we want. We should also open up the logger to users of SwiftCrossUI so that they can re-route and/or filter SwiftCrossUI's log messages. This would also allow users to upload SwiftCrossUI logs along with their issues easily.

This would go well with #248 which discusses the possibility of providing users with the option to upload SwiftCrossUI error logs to a central SwiftCrossUI server.