Create macOS universal app bundle

[dgavedissian]

To be able to distribute a macOS version of the game, similar to the win32 distro package, we need to create an app bundle as part of CI.

My suggestion would be to assemble an app bundle as part of the build.sh script, probably using steps like in this script: http://bazaar.launchpad.net/~widelands-dev/widelands/trunk/view/head:/utils/macos/build_app.sh#L47

We should also add support for cross compilation, so we can build both an x86 and arm64 binaries from the macOS CI builder, then use lipo to combine those binaries together (see rust-lang/cargo#8875).